!ask if i target my unit at Sandswept Tomb with Rampage and want to pay the additional cost for +2 might, does Sandswept make the +2 might discounted to be free instead of costing a power?

Yes — Sandswept Tomb's discount makes Rampage's optional [C] additional cost free, and you still get the bonus.

You must still declare you're paying the additional cost when you play Rampage, but discounts can reduce additional costs to 0, and an optional additional cost counts as "paid" if you made the decision to pay it — regardless of what you actually paid (Rule 356.4.f.1; same as the Clockwork Keeper example). Since your friendly unit at Sandswept Tomb is a legal choice for Rampage, the spell qualifies for the [A] discount, and it eats the [C] cost entirely. One correction: Rampage gives +2 [S] (Shield) this turn, not +2 Might.