This is a great topic, made me think about what I've done and the plants I've lost as they've become learning tools, hehe.
I think in a decade of playing with aquariums, I could easily say I've lost maybe $100-200, but that figure is only really that high, because before I knew better. I was buying plants, that were not appropriate from sources that should not have been selling them in the first place. Buying from fellow hobbyists and being lucky enough to find local sources to collect from has really saved me money. And with a little bit of time, I've been able to propagate most plants, so a few sprigs go a long way too.