What can I expect to pay to have my rear end rebuilt? While its open and such Im going to put in 3.73's also.

I dont have the money for a 12 Bolt or 9" Or I would go that direction.

 

I don't know about labor price, since i have always done mine myself. I wouldn't say anymore than $2-300 for labor.
I just rebuilt mine, my install included:
Precision Gears 3.73 ring and pinion
Eaton Posi
Solid pinion spacer
Summit rearend girdle
Install kit (pinion bearings, carrier bearings, shims, races)
Limited slip additive
Gear oil

I paid around $700 for everything, but since i did it myself, i saved a lot in labor. Good Luck.