If you forget to negotiate with them, and you go with O'Reilly, they are really good at sending free books for book signings to conferences which can be a really great chance to both get the word out and meet people that care about the tech you wrote about.

100 to 300 tends to be reasonable

When book sales are split between multiple parties I can't imagine it's okay to get a bunch of free ones and sell them yourself, keeping 100% of the proceeds.