Agree with GrumpyWell I do agree with Tinner, though I have repaired and restored numerous 45 mil ballasted roofs. It's a 10 year rated roof and a ridiculous investment, but they are out there.
So your question of how much it should cost can not be answered. There is nowhere near enough detail. How is the access to the building? can it be vacuumed off or does it need to be manuallyr emoved? How tall is the building? What kind of insulation do you want? Material and labor prices vary regionally. How do you want the new EPDM installed (mechanically attached or fully adhered or ballasted?) So many flat roofing options so little time.