First pattern discovered!

It's sneaky because it's not clearly visible when you look at the result.

I didn't know each item had one until I happened to notice it while checking item prices for some of the unknown item values (for egos).

There's nothing sneaky about buy price. It's visible in game for almost anything you can buy from npcs, and for everything else, we could still get it without data mining by deriving it from everything else that stat regulates(time on ground b4 it poofs, repair costs, lost and found retrieval cost, bank and mailbox fees, ego food, and possibly campfire burn exp) We've known about what else is determined by buy price about as long as the game has been out. ID on the other hand isn't visible anywhere ingame, and so far we have no way to find the exact number without data mining.

All those methods would give you the price depending on its "current price", not the "base price", not to mention some of those methods giving inaccurate results (disappearing time).

As Kevin mentioned, the actual "buy price" isn't actually visible anywhere, since the only visible one is a multiple.

More to the point, the reason I said it was sneaky was because the data's in the wiki, but not actually printed out.

It is, if you hover over the NPC resale price...

Well, score another feature that most people don't know about because it's the ONLY cell that does that.

Because it was for testing purposes.