If its just a conversion, thats changing more than needs to be changed.
All you really need to change for r-134 is expansion valve (not everyone says you need to change it) and the stuff in the lines, and possibly the lines (if they are iffy).
now if the system is broken (like mine is) that could be the right price. My compressor has been going bad for a couple years and ifnally went. I pulled the lines off to check how bad it was and my system died the "black death". There is black stuff (lots) throughout the system.
In my case I need a new compressor, new hoses (they are old anyway), new dryer, expansion valve (actually, I think its an oriface tube on the x).
The Dryer is <$100, compressor was about $200, hose was about $100ish, oriface tube <$50. If I had a shop do it all, at the normal rate of $70/hour it would be a good $500 in labor and those prices I listed are Autozone prices. Shops don't charge autozone prices. They charge MSRP. I checked at my neighbors shop and the prices I listed above are about 1/2 to 2/3rds of what the shops in town would charge for the parts. Soo, if there is a reason to change all those parts (other than to just be safe) then the price is right for a shop to do it, but in general, you don't change all that to convert to r-134.
~Mark