Somehow I always felt bad about going to a small mom and pop FBO, sitting around all day, watching only a couple other airplanes come in, buying no fuel and paying no landing fee. Sometimes I'd even offer to pay one. But I can be generous with other people's money.
I can see how FBOs need to charge for their services especially if you stay longer than an hour or so. You're there taking up space, drinking their coffee, eating their cookies and using their crew car. A lot of places waive the fee if you buy fuel.
I think a reasonable fee for a small piston airplane if you buy no fuel and stay for a while would be $5 to $10. For a turbine maybe $25 to $50. That said, I've paid as much as $400-$500.