If my opponent uses Hard Bargain on my Rampage and I paid the additional cost, do I get the +2 might?

Yes. If you pay Hard Bargain's [2], Rampage is not countered, and since you already paid Rampage's [C] additional cost when you played it, the +2 Might is granted when Rampage resolves.

Sequence: Hard Bargain resolves first — your [2] payment means its counter effect does nothing. Rampage is still on the Chain, so it resolves normally and your friendly unit gets +2 this turn, then both units deal damage equal to their Mights to each other.

One distinction: Hard Bargain's [2] only stops the counter — it has nothing to do with the +2. The +2 comes only from Rampage's own additional cost ([C] Body), which you pay as you play it (Rule 356–357). If you hadn't paid that, Rampage would still resolve but without the +2.