Firstly, you don't have a VWP for 2 years, you have an ESTA. That is what is valid for 2 years. The VWP is the 90 day visa-waiver you get as you enter.

Will it reset if you go to Costa Rica? Yes, yes it will.

As seen in many questions on the site, if you leave to go to Canada or Mexico, it does not reset. The 90 days keep on ticking.

However, if you travel further afield - Central America, UK or otherwise, then it resets.

The official list of the countries where it DOES NOT reset, is available in this question. Costa Rica is not on this list.

As mentioned before, however, you still need to convince the border guard that you aren't just doing a 'visa run', and you achieve this by showing your itinerary, evidence of your flight out of the US, and if you want to be extra certain, evidence of your life back at home, to prove you're merely visiting, not intending to stay.

Firstly, you don't have a VWP for 2 years, you have an ESTA. That is what is valid for 2 years. The VWP is the 90 day visa-waiver you get as you enter.

Will it reset if you go to Costa Rica? Yes, yes it will.

As seen in many questions on the site, if you leave to go to Canada or Mexico, it does not reset. The 90 days keep on ticking.

However, if you travel further afield - Central America, UK or otherwise, then it resets.

The official list of the countries where it DOES NOT reset, is available in this question. Costa Rica is not on this list.

As mentioned before, however, you still need to convince the border guard that you aren't just doing a 'visa run', and you achieve this by showing your itinerary, evidence of your flight out of the US, and if you want to be extra certain, evidence of your life back at home, to prove you're merely visiting, not intending to stay.