At least my experience, in liquidating donated books to raise money for the RMRRC's grants, is that it really depends on what you've got. There's a few good ones and a lot that you'll be lucky to get $10-15. But if you price them right, you can move a lot of books at a train show. At the Denver show last March - right before everything shut down - we sold probably 5-6 boxes of books despite lower show attendance than normal. If you're okay lugging them to 3-4 shows, you might have luck getting better money, but for us it was about what goes into the show doesn't get lugged back out.
Even then, the "few good ones" aren't fetching near the money they were a decade ago. I paid $150+ for my copy of Silver San Juan fifteen years ago, and now you can find them easily for <$50. Same with Mac Poor's DSP&P book. I think the last one I hauled to a show sat there all day at $75 until I marked it down. The used railroad book market isn't what it used to be by a long shot. That's good for me - still being very much in the acquiring phase - but not so good for those who hoping to sell their collections for what they were bringing a decade ago.