It's a low amount and either they keep you or not, you will be in a studio that has amps and hardware, anything you need to play properly and this has a cost. If share is equal it's fair and it's musicians playing together. If you get a good time, and they do too, well, it starts there. That money goes to the studio for the service offered. Now, they could invite you and you would not pay a thing of course, and in the end, you make your own decision. I would pay for the service from the studio to help me play in the best conditions possible.
Some do consider you only start to pay once you are officially in the band. I see it in a different way. I see it like a kind of "investment" by sharing equally as if I was already a member of the band and start playing like it's the band I've chosen and see how it goes (and it goes both ways).
When I'm invited by friends, I bring something. Either food or drink, but I like to bring something to participate. Make them taste something I love and share the experience.
Now. If you consider you should not pay unless you are part of the band, well, don't and explain them why. When I do such auditions, I'm not paying the band, I'm paying the studio and will have a chat with the sound engineer to see if it's a place to add to my contacts for studios I might bring friends to.