Varies by area, day of the week, and type of "lounge." If you are playing strictly background, no vocals, no interaction with the audience, I wouldn't expect or ask for more than $60-$85 per night. And if you aren't taking requests, playing popular things, special event type things (i.e., favorite song for anniversaries, happy birthday, ) expect little in the way of tips.
Now, if you can do all these things and really please the audience--no longer background--tips can run $25 --$50 per night.