PPro-
Forget what everyone else is charging! Forget what the builder is offering to pay! The only thing that matters is how much money you need to charge to cover your costs (including your salary) and turn a profit.
Though you say you haven't bis work before, I'm assuming you know how long it takes you to hang rock. The next thing you need to do is determine how much rock needs to be hung. You need to do a linear foot takeoff of walls, multiply by the wall ceiling, and then add the ceiling square footage. Let's say, for example that all totals 3,000 SF of rock (most simple homes are in the +/- 3 SF of rock per SF of floor area- though that can vary widely).
Assuming you've determined the total SF of rock, and you can determine how long it'll take you to hang it, you now need to determine how much to charge for your time. Do you have insurance? Truck payments? Advertising costs? Any other business overhead expenses? How much do you need to make after covering all of those costs?
Based on our 3,000 SF example, let's say it's going to take you 5 days to hang it all. If your overhead expenses are $100/day, and you need to make $300/day (don't forget you need to pay both sides of FICA, etc- so factor that into your daily rate), that means you need to charge a minimum of $400/day, or $2,000 for the five days of work. You should also be shooting for a minimum 10% profit, so you'd charge around $2,300. I don't have my calculator in front of me at the monent, but that's somewhere around $0.75/SF (coincidentally). Use your actual rates and arrive at your actual price you need to charge.
SInce your just starting out, you may as well do it the right way from the beginning. NEVER let anyone else dictate how much you're going to make on a project. If you need to charge $1.00/SF, tell the builder that- if he doesn't like it, let him find someone else.
BTW- it was nice of him to offer you $0.10 less that his normal sub who wouldn't take the job because it was too far out. Nothing like paying less for a job farther away. Some of these builders are such scumbags......
Bob