Typically to determine source of an air conditioning leak; the preliminary estimate is around $200; this includes an evacuation and recharging of the a/c system including charging the system with a ultraviolet dye to help determine source of the a/c leak. Once the system is charged, the vehicle may have to be driven for a couple weeks time period if the leak is too small to determine the source at that moment. Once the air conditioning stops working again (aka all the refrigerant has leaked out); the source of the leak can now easily be determined because of the dye that the system was charged with.
Yes, repairs can reach near the $1500 range depending on the component that is leaking. An evaporator replacement would entail having the dash panel removed; which is several hours in labor, and an a/c compressor can cost upwards of a $1000 for the part alone. The most common leak though is the a/c condenser, which is mounted in front of the radiator and is typically damaged by rocks that kick up off the road on the highway. This replacement usually ranges between $600- 800 depending on the part used and labor rate of the shop.
Hope this all makes sense and helps. If the shop is charging you $1500 up front just to find the leak, I would look for a new repair shop. Please use the accept button as this is the only way I receive credit for my work.