Generally, you can't count on more than a 25 year life out of a flat tar and gravel roof. If your roof is close to that age, it DOES make more sense to replace the entire membrane than to keep spending money on repairs until you finally cut your losses and have it replaced.
The cost of the work will depend on what you have done. Typically, a flat roof will involve stripping off the old gravel and built up membrane. Then stripping it right down to the roof deck, or at least stripping off the top layer of insulation board to ensure they have a clean dry surface to start with.
Typically, when you install a flat roof, you will strip the old roof down to the wooden roof deck and replace any rotted wood,
Then you put your extruded or expanded polystyrene insulation down on top of the roof deck. It's actually a good idea to opt for a "sloped roof" rather than a "flat roof". On a flat roof, then they just put down the thickness of polystyrene insulation you specify. On a sloped roof, they make a map of the roof with any roof drains on it. Then they feed slabs of insulation into a computer controlled cutting machine that cuts the tops off those slabs in two dimensions at the same time with a hot wire. When the cut slabs of insulation are installed in the correct order, you end up with a 1/4 inch per foot slope in the roof to the roof drains.
The main advantage of a sloped roof is that if there ever is a roof leak, the damage will be minimal because the water will drain off the roof into the roof drains rather than puddle on top of it.
Over the insulation you put fiberboard panels. You want at least two layers of fiberboard panels and have them staggered at the seams so that the 300 deg. F tar won't seep through at the joints and melt the polystyrene insulation below.
Over top of the fiberboard panels they "build up" a roof membrane. This is done by rolling out 4 foot wide rolls of roofing felt paper into hot tar, which bonds each layer of paper together. On my building's sloped roof, they used 4 layers of paper in the membrane, plus another 4 layers around the perimeter of the building. I don't know why they used the additional layers around the perimeter.
Finally, once the membrane has been built up, they mop hot tar onto the paper/tar membrane and broadcast clean pea gravel (typically 1/4 or 1/2 inch clean) onto the roof membrane. The purpose of the pea gravel is to protect the membrane from the intense UV light from the Sun which would deteriorate the membrane over time.
In my opinion, you get the most bang for your buck if you're intending to keep the house a long time by going with the sloped roof rather than a flat roof. On my building, which is in an "L" shape with outside dimensions of 100 feet by 100 feet, I paid about $25,000 for a new sloped tar and gravel roof in 1991. I have had no problems with that roof so far, but it is nearing the end of it's lifespan.