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Foil Delvers were a great spec. I ended up with 2 sets, should've bought a lot more (I ended up with 4 foil sets of Invisible Stalker, which just seemed better). Regular Delvers are still a decent pickup for 75 cents or less. Even though they're a common, they're not common common in the flip-card spot, and they're likely not to be reprinted for a while, if ever.
Even aside from regular Standard-legal sets, the boxed/special sets can take values way down, especially since they are doing more open-ended printings like Commander and Conspiracy. But the flip cards are likely safe.