Referencing my collection via Discogs, I think the Classic Records 180g issue of The Who's "Tommy" was my most expensive single purchase, around $700. I have box sets that would now go for much more, starting with "The Beatles In Mono," Gentle Giant's "Unburied Treasure," and CCR's "Absolute Originals."
I have a lot of Classics, more Who, LZ, Genesis, etc. Ditto tons of Music Matters jazz reissues (which I particularly love). And for better or worse, about a dozen MoFi One-Steps (Bill Evans's "Sunday at the Village Vanguard" is worth a fortune; holy smoke!).
Fortunately, it's all covered by my homeowner's policy. You better believe I made sure of that years ago.