They can put a dye in the AC system and with a black light can tell exactly where the leaks are. Often times the leak is a compressor seal, and that may be an easy fix...or it might be better to just replace the compressor.
The bulk of the price for the repair is going to be the labor since they have to evacuate the system, replenish the compressor oil and recharge the system with refrigerant, on top of repairing or replacing the part/s that caused the leak.
It's hard to have any AC repair done where the cost is less than $400...but my real concern is that they just said your "Main line" which is not very descriptive. The main line can consist o every component in the AC system since it is a recirculating system....Some are metal pipes, and some are high pressure hoses. it could even be an evaporator core, condenser core or the compressorwhich can get really expensive. Like Yardsale said, you need to be more specific..or it would be better to say, the repair shop should be more specific.
If they cannot pinpoint the exact location of the leak, then I would walk away and go to another shop that can.
...Rich