$800 would be cheap for that job as it's normally about $1100 here. The part is about $300 with freon, dye and o-ring and about 9 hours labor to replace it (not including diagnostic time).
One shouldn't guess at that part either so I assume that dye was previously added and that they actually then dye checked your evap with a black light, seeing the leaking dye and not just assuming? If not, get another opinion and preferably from a shop (dealer) with actual experience diagnosing evap (or other freon) leaks.