If I'm filling in for a band, I name my price. If they can't or don't want to pay me that much, they can call someone else.
If it's my own project, I often play for no money at all. We do a lot charity gigs. Hell, sometimes if I like the cause I'll even play for free and offer a donation. But if I'm filling in for strangers, I have my price.
The problem I run into with this is that if the price is too high, I have to take it out of my own share. And I've done it when I've had to, but I don't like it at all. I only call that guy when I have money on the table and I need him....