What kind of seed? What is the rate and quality of the seed and its cost? Rye? Blue? Tall fes? Slit seeding? Aeration and broadcast? Power rake and seed? No prep and sow seed broadcast?What about the risk of non-payment?
What about the expense of callbacks? What about if customer does not water--but wants you to guarantee the job? What if weeds come up? What if poa appears?
A lot depends on the cost of the job, before adding a margin for profit.
You should plan to charge enough to repay your cost of equipment and its maintenance within a year or two. Add your labor, insurance, and contingency for unforseen problems. Write a tight contract to protect yourself from callbacks...and get the money before you leave the site.