I've tried having this discussion with other people and there are many differing answers.
You won't get a definite answer and for good reason. There's many different people out there, not everybody has the same kind of money to spend, and not everybody thinks the same as others do.
Obviously one dollar seems like a waste of time (even though all donations can help) and let's be honest but no one (probably) is going to donate 100 bucks in a single donation.
"Reasonable" is very subjective; depends on the person's budget, if they even can purchase things online, and what that person considers "reasonable", or if they are generous or very cautious with money. :)
Btw, I've seen donations in the thousands from various members on other forums, so because you find a dollar a waste of time, and 100 a ridiculous amount, doesn't really mean that it will be that same shared opinion with everybody else. Some may consider that 100 dollars of the same value for what you would consider a dollar, and some would consider a dollar the way you would consider 100 dollars...
Because of this however, and because you'll always get a variety of answers, you really have to answer this question on your own, based on what you think is reasonable. You know your personality and budget better than anyone else does in my opinion I think it's safe to say. In all honesty I don't see how you'd expect others to dictate that for you (which is essentially the bigger picture here if you think about it).
Note: The way I see it you've already defined your own scale... $1 - $100. So if you are a generous person, perhaps you take 70% of the higher limit and put in $70. If you just want to pitch in, perhaps it's only say $10, or maybe you split it in the middle, but regardless that is what you decide, not anybody else.
I think I donated about $10 or $15... In my opinion, and because I have an idea of what could be done with that amount, {I} thought that this was a reasonable amount, because it's a fair sum that *perhaps most* people could spend, yet it's sufficient enough to make a difference if it was to be put into domain registration/renewal, hosting, or even forum upgrades or advertising...