Always get a second opinion, but it really could be that high. Depending on your location (hourly labor charges vary) the labor could be something like $65 and hour. On many cars this valve is located in the dashboard, in a box, connected to the a/c cooling lines.
So this basically means, taking the coolant out of the lines with a recovery machine, taking apart the dash (in the area around the glovebox) and taking the part out, replacement of the part, pumping the system down to a vacumn to get any air out, and then refilling the coolant.
This is a pretty intensive procedure, but I really cant figure out how the mechanic would determine that it was the expansion valve from what you wrote. If you tried a recharge kit, did you disconnect the battery for a while? There is a low pressure switch that keeps the compressor from starting, some of these have a memory. If you disconnect it the bat, and then try starting the a/c it should work for a minute, and then you might see if recharging makes a difference.