Very hard question to answer due to the many variables from size, extent of damage, what is under the membrane, to possible water damage. Of course your location is also a factor.
Having had to look into a roof repair recently, I can tell you it is the professional labor that costs the most. The parts not so much. Most RV garages around here charge well over $100 an hour.
So, a complete replacement done properly around (pretty expensive) here will run you anywhere from $3000 to $5000 on a 25 ft class C. It could easily go much higher with rot and water damage.