Cheap flights from Heathrow or Gatwick to Juan Santamar铆a International Airport, Costa rica?

Im going to Costa rica in July to do a turtle conservation project Im currently a student at plymouth university so I need to find the cheapest flights from Heathrow or Gatwick to Juan Santamar铆a International Airport Costa rica from-

Sat 5th July to Mon 21st july

I can only find ones 拢650+ you would be a star if you could find one even cheaper.

Appreciated

Laura xCheap flights from Heathrow or Gatwick to Juan Santamar铆a International Airport, Costa rica?
I used the web site below last year; found it as good as any. Maybe a bit early to get the cheapest rate. I flew with Iberia, via Madrid, which has less security hassle than via the US route. There seems to be some direct flights now; did not seem to be any last year.