This seems pretty awful.

I originally thought about it hitting cards lands. I'll admit I'm having a hard time creating lands that might not be lands.

If you're going to do lands that sacrifice them for an effect, you really want it to be under the player's control. Otherwise, players will hate them. What happens if I really want this for the mana? You'll mana screw yourself constantly with this random effect.

Sacrificing isn't necessary, but the land should leave the battlefield in some way. I thought the flip effect was an excuse for the land to enter untapped, since you weren't guaranteed to get any mana out of it.

I suppose I could rework the card so that land would be sacrificed at the end of the turn. That way you still get a mana for that turn. Would the risk of only having the land for a single turn justify allowing a dual land to unconditionally enter untapped?

You can always respond to the trigger by tapping for mana anyway, so there's no real difference in immediate vs. delayed sacrifice.

The wording also curiously uses the term "call correctly", as though it's somehow superior for you to get a one-shot mill effect and no land, and as though you somehow have the option to refuse to make any call at all (thereby guaranteeing that you do not "win" the flip).