sandswapt tomb if I use rampage targeting a friendly unit in sandswapt tomb. The 1 power is free?

Yes — the additional power cost is free, and you still get the +2 bonus.

Sandswept Tomb discounts any spell that chooses one or more friendly units there by [A], and because you're targeting a friendly unit at the Tomb, Rampage qualifies. That discount eats the optional power additional cost entirely (you still have to declare you're paying it — an optional additional cost counts as "paid" once you make that choice, even if it gets reduced to 0). The 3 Energy is not discounted, so you pay that normally, and the chosen friendly unit still gets +2 [S] this turn (FAQ #11846; Rule 356.4.f.1).

Same logic applies to any other optional power additional cost on a spell choosing a friendly unit there.