I mark things up at least 15%. Spreading 50 yards of anything, even with a machine, is going to take some time. I've never spread river stone like that but have spread plenty of driveway stone. I'd probably budget 2 days for the job to be sure I made money on it. I can't imagine it would take longer than that.
What about trucking cost?
Are you subbing out the killing of the existing grass?
Considering your material and equipment cost is $1,815 and your labor cost is $250, $2,500 seems pretty low to me. Remember you're trying to make a profit, not just cover your costs!