$1200 Evaporative Core...Excessive???

Our '02 TB's A/C wasn't working well, so we took it in to the local Conrad's. They dye tested it and saw no obvious leaks. We brought it back several weeks later, as the A/C wasn't working too well. They said that they found a leak at the Evaporator Core, and to fix, it would cost $1200 to fix (half for the part from GM, half to pull the whole dash. Does this sound right? We've had good experience with this shop, but the repair seems excessive. Thoughts?

The labor in the Mitchell guide is 3.5 hours, plus leak test and charge and the parts, seems high to me. Unusual failure these days, I would insist on a guarantee that this fixes the problem.