I do service AC as side job and have rescued few units from junk yard, but it is tricky job.
When I DIY my own unit, I install a valve and charge it, what cost about $15, but the point is that knowing the unit lost the charge once, the risk it will loose the charge again is pretty high, therefore you don't want to give warranty for such fix.
My repairs last me for over 2 years, so adding $3 of refrigerant later with my free labor is good deal, but once you pay the technician 3-digits rates, it very fast make no economical sense.