I have used a tool for a while, mainly because I'm in awe of how its built. And funnily enough its called Kayak.com
Check out their Kayak Explore - see where you can go for how much
Its a guide to show you how far you can get on a budget at different times of the year based on real data collected form hundreds of airlines (including Ryanair, but make sure to confirm the prices as it might not be up to the minute).