Some examples from the last race at Bathurst (R6):

Tatts $26 Betfair $516 this is about 19 times the tatts price
Tatts $59 Betfair $422 this is about 7 times the tatts price
Tatts $61 Betfair $463 this is about 8 times the tatts price.

Maybe 500% was a little too conservative.