Why not just bet to collect $100 at the "rated" price.

So in PP's example

Bet $25 on 3/1 ($4.00) chance. Collect 25 x $4.50 = $112.50 (if wins)
Bet $12 on 7/1 ($8.00) chance. Collect 12 x $9.00 = $108.00 (if wins)

If 3/1 ($4.00) chance wins, outlay $37 ($25 + $12), collect $112.5. Win $75.50