I know that you can find better deals on Mintbase on NEAR protocol than on opensea on Ethereum. A NFT rock on Mintbase goes for 50usd vs 3 million on opensea.
It seems like that you're missing the point. The problem is why an NFT picture gets overpriced like that. If you're talking about the comparison of the price will not be matched with what uniqueness, prestige that you will be getting it by owned a rare NFT on the opensea.
Just look at that VISA was also buying crypto punk NFT. It's about the uniqueness combined with the hype. You can also get NFT rock even cheaper in unpopular NFT marketplace that was operated in the different blockchain other than Opensea and mintbase.
The true value of rare NFT just on its limited edition and uniqueness. it doesn't have any others utility usage but some NFT may offer the ability for the owner to stake it to get benefit from what they have owned it.