Some of the old D&D adventures are classic memories of the games of our youth. Isle of Dread was my first actual game session experience. It seemed fitting that our characters were exploring the world, just like the players were first exploring the game.
That, as the saying goes, was then. This is now.
Now, adventures are something for most RPG publishers to dread. They want to publish them. They might enjoy writing them. They almost certainly love to play them. Unfortunately, adventures don't sell very well.
Depending on the product, the size of the product line (the length of its "tail"), the rate of product releases, and the publisher's overall business plan, a published adventure might see 10-30% of the core book's sales. If publishing an RPG is a risky business, publishing an adventure for your game is just plain stupid. You know it can't earn a large profit. At best, you hope it breaks even.
The worst part is that at least one adventure is crucial in helping to establish a game or a campaign setting's tone. Arguably more so than any supplement, an adventure shows people how the game or setting is meant to be played. A certain vocal element of the gamers playing those games demand these products. In order to meet this demand, some publishers offer adventures as a separate product.
Also, while the adventure might not sell very well, the release causes a spike in core book sales. Adventures do legitimately attract some new customers that previously weren't interested in the game or setting. More importantly, they focus attention on the product line as a whole.
In fact, this concept was the entire reasoning behind the release of the D20 license. Let small-press companies publish adventures and lower-budget supplements and WotC sells literal tons of Player's Handbooks. So while the adventures don't directly put cash in the WotC bank account, they do so indirectly.
So you see the publisher's incentive and disincentive to publish adventures.
Two criteria are always important for a good freelancer. The successful freelancer always thinks about money and marketability. Ultimately, marketability is only important because you can turn it into money.
The publisher might very well offer the same word rate for an adventure as he does for any other written work. In fact, no publisher I know of offers different values for different types of work. The rates vary by your name recognition (that marketability thing) and how easy you are to work with (which they don't know until they work with you once).
So let's say I've been foolish and have done all of my work without lining up something new. I have two proposals in mind that I like and want to spend my time developing. One is a race supplement for a cool new RPG. Let's say it's a nezumi book for Legend of the Five Rings. The other is an adventure. Since we’re talking about L5R, let's stick with that. The adventure is set in the Shadowlands and center on the nezumi.
"Race Book Nezumi" pays 3 cents per word. "The Five Burrows of the Nezumi" pays 3 cents per word. Which should I write?
I write "fluff"—background material, history, and adventure ideas—rather quickly. When I first start out, I can maintain 3,000 words per day, most of which I’ll keep after editing. The amount of "crunch" these products requires (stats, feats, spells, magic items, space ships, superpowers, etc.) is small, which keeps my relatively unproductive days few in number. By the end of the project, I’m slowing down, but an overall rate of 2,000 words per day is normal. Adventures, on the other hand, require a large number of stat blocks. Stat blocks, for those who haven't tried to write a long adventure, suck up large amounts of time that I'd rather spend with an auditor or a dentist.
Adventure writing might progress, after editing, at 800 words per day. Adventures require playtesting, and they really require blind playtesting by a third party. If you have to wait for one of more groups to playtest "Five Burrows of the Nezumi" and comment on the adventure, you're adding delay before payment, but not time that you spend working. You do have to review their comments and weight their impact before making any changes, then make those changes.
So, to summarize thus far: Race Book Nezumi makes the publisher more money and pays me $60 per day at this example word rate. Five Burrows of the Nezumi makes the publisher less money and grosses $24 per day.
Marketability is the other element to consider when writing. Many writers talk about exposure or name recognition. While being recognized by the readers might be emotionally gratifying, its financial relevance lies in its ability to land you a new job or to negotiate better terms for work your queries get you.
Again, the impact is considerable. A mid-tier publisher might sell 2,000 copies of an adventure. Actually, that's not bad; many sell less. Of the ones that the company sells directly through website and conventions, 100% reach gamers. Let's say that 30% go to the book trade, and only 25% of that number are eventually returned. Of the copies sold through game distribution, we'll suppose a generous 80% of those reach store shelves and 80% of those sell to customers.
Gamers own 1,418 copies of this print run at this point. You might be able to name the writer of every product you own. In all likelihood, you can't. You know the names you see the most often. You might pay attention to the name of a favorite writer. Just like you, the owners of those products might not remember who wrote them, especially if the author's name doesn't appear on the cover anywhere.
Adventures are useful to game masters only. That cuts a large fraction off of your potential sales already, but that impact is already figured into the numbers previously discussed. It does mean one other thing, though: the next book you write, if it targets both players and GMs, receives fractional benefit from any name recognition.
Another factor figures into name-recognition. A gaming group might use a setting book forever, a regional sourcebook for the duration of a campaign, and a class book as long as they play a particular class. A game master pulls out an adventure once, runs it, and puts it away.
These factors combine to trivialize the name-recognition impact of any single adventure you write.
I'm not arguing whether or not the game industry or a particular game needs published adventures. If a staff writer cranks out one or two a year, and the publisher absorbs the cost of having a freelancer do one once a year to provide a full product list to the customer base, that's great. Everybody’s need is being met, and I'm happy.
My assertion is that professional freelancers who are trying to earn a steady income from writing in the RPG industry should focus attention toward other products. Adventures are not a top choice for earning money or lining up future projects. Find a more rewarding project.

