The tricky thing is if you create a bass line that is central to the tune, should you just be paid for your work or get a co-write? I've seen it go both ways. There are bassists who when they see that they are creating the musical vision stop and talk to the Producer and negotiate a co-writing credit. If you're not American Federation of Musicians, then you could get co-writing credit in lieu of payment, but you'd have to get paid the recording scale if you're AFM.
If you're not, it can go anywhere. I've seen guys work for as little as $25-50 a track or as much as several hundred dollars. It's all negotiable at that point. Personally, I'm AFM because it gives me a starting point for session scales. Here's the Nashville Session scales, for example:
Nashville Musicians Association
As a player, I'd advise you to make sure your bass doesn't have any buzzes or rattles, that the electronics are clean and that it plays in tune. A lot of time in the studio they will take your bass and plug it into a DI rather than your amp. They can then run it into a compressor/limiter (sometimes that's done "in the box" on DAW software like Pro Tools), studio preamp, etc. to give a consistent signal to the recording medium (mostly computers these days). IME, it's a good idea to have a couple of basses available, perhaps one "Modern sounding" active bass and one "Vintage sounding" passive bass with flatwound strings.
Good luck with all this! Check out online interviews with Mike Elizondo, Leonard "Hub" Hubbard, Owen Biddle and Stephen "Thundercat" Bruner among others for some more specific musical perspectives for cutting Hip Hop and Rap bass.
