The featherweight UK champs are held at the NEC every April. The fighting can be brutal, but there's no better way to learn than to see which bits of your robot get chewed up!

A typical featherweight, depending materials/construction methods/design, could cost anything from £150 to £1500. I'd guess the average range is in the £300-600 sort of area.

I'll let others with experience comment about the other classes.