I have a small collection of RFs and I didn't pay much for any of them. Well, a couple of them were given to me.... 🙂
I got an Oly RC for something like $10, as the meter has a flaw; I think they originally wanted $60 or $80 or something like that, for what was a bit of a beat-up camera. I figured for $10, it was worth it to play with. It actually works better than it looks. 🙂
I got a Petri 7s 1.7 for around $20 including shipping. That's a really nice one, I think, but quirky. It comes in both 1.7 and 2.8 lenses.
I've seen Hi-matic 7/9/11 go for auction in the, let's say, $25-50 range, so I think that's certainly doable to get one of these cheaply. The Olympus Trip is popular, if you can live with f2.8 and zone focusing; I used one of these for years (before I was more interested in photography).
I think there's a lot of decent fixed-lens RFs available at really good prices. If you want the most popular or the best ones, you will pay a lot more. For example, the Hi-Matic 7sII is highly regarded, but really expensive. But noone knows Petri, so it goes unnoticed. I think part of the price is driven by size; some of these models mentioned (Konica S2?, Minolta 7/9/11, Petri 7s, etc.) are physically large. Also, if you can fall back to a 2.8 lens, things will be cheaper. I think I paid all of $20 inc. shipping for a Canonet 2.8 and gave that to my sis. It's not the best, but not bad. I think I prefer the trip tho.
My beef with auction sites is that the condition is unknown. The sellers often don't know if the camera works, or if they are more knowledgable, the price is often higher. It's a bit of a risk shopping this way.
Good luck!