Absolutely disagree with TFuto, so wanted to chime in as well. The photo is not just your work, but more importantly your product. They are the buyer and you're the seller/supplier. You're standing in a world where everybody can make photos, so the supply of photos is incredible great and on top of that there is (nearly) no cost 'per copy sold'. Point in case, the competition is pretty harsh. So the question becomes, how unique is your photo? Do you believe they can find somebody else who would offer them a photo they can use for a lower price? If so, take the $50, otherwise you can always try to make a counter bid. (And my advice would be to take it, check photodune.net for typical rates at which photos go nowadays (it's a lot less than it used to be)).

Just, don't ever ever do anything like

If they immediately come with an unfair response, I increase the price e.g. 10% or more. I insist being paid and my product being properly priced.

A counter bid is a counter bid, you're part of an economy and discussing prices is a part of that. It's not about honor or self-worth, it's just business. That might sound harsh, but honestly, you're better of being fair and polite in business than having an inflated dose of self respect.

Btw, just an afterthought, putting up your sellable work on a site like the aforementioned photodune (or more expensive site if that fits your... alignment) might both spare you paperwork and get you a nice extra income if you're doing this as an amateur.