Your leak might not cost much to fix. If you're lucky you just have a leaky o-ring or schraeder valve. I would get it fixed the right way if you want it to work as expected. The a/c components, the compressor in particular, are rather delicate and very expensive parts, so I wouldn't put anything in that might jeporadize their lifespan. If you want to take some time you can lean how to fix a/c yourself. I did.
'84 300D, 173k
'90 300CE, 162k