Yeah, some sets can be ridiculous money, especially if they're retired sets that are hard to come by. One vehicle I have is worth a fair bit, but I'm thinking of keeping that one, as it is fairly rare on top of the cost.

Some second-hand kits seem to be cheap, but sometimes come disassembled, which would be a major pain to sort, and construct, but could be worth it. Even if they're missing pieces, you can actually get the manuals online, and order particular pieces that are needed.