I just had an estimate of $550 Canadian to replace my leaking compressor, labor & freon included. Does this sound right?

Sounds cheap to me. 134 or R12 refrigerant? I personally do not like rebuilt AC compressors. But, if the shop will garanty the job, go for it.

Most compressor manufacturers will not honor their warrantee unless the installer purchases other expensive AC components along with the compressor. Marketing genius. This adds to your cost. You didn't mention any itemization so it's not easy to tell what will be happening.