when is the cheapest time to travel to Costa Rica, for a business appointment.?

Right now, or at mid year. Because in November-December-January is when all the tourist come, so the prices go up.

the best deal at the moment is with britsh ariways for £903 and thats during the school time when onones on hoilday so it's going to be cheaper then

The rainy season, or winter in Costa Rica (May through November) has been dubbed the 'green season' by the tourism industry.
Our first green season traveling in Costa Rica we expected the worst. At least a couple of hours of rain every day, and a few weeks of solid rain. We were pleasantly surprised that over half the days it didn't rain at all and the rest of the time it was a couple of hours in the afternoon bracketed by brilliant sunshine. In more than three months there were only two stretches of three days when it rained most of the time. During these drizzly days, it was clear and sunny on the opposite side of the country, so we could have avoided the rain had we thought it was a real annoyance.All prices were 30%-60% lower than the high season and Our airfare was only $245 from New York
If you are thinking about a trip to Costa Rica during the green season, don't let the weather stop you.

try Spirit Airlines, joint their club they e-mail you last minute deals