Steve,if you just look at the finance side its going to cost a lot more than £100 per race.
new 620 monster fully prepped £5000,assume sale at end of season £4k.
So assumming interest of 5% =£250.
Loss on bike £1000+£250= 1250 8? races=£156per race
Add to that tyres,fuel etc (without entrys) +labour to prepare bike i'm sure you would be on the wrong side of £350 per race.
But then if you look at it i suppose £2500 for 8 "turn up and ride" races proberbly isnt that bad.
Then again add entry fees etc and its not so good
Butch