A quick check reveals nothing wrong with your numbers UB.

I checked my figures from Last Saturday to tonight, I have bet on 80 races and averaged just over 102% per race, so in your example add a runner at say 30, that's an extra $3.33 in per race, times 100 = an extra $333 in, added to your loss of 13 = +320. Not to be sneezed at as the downside is limited. Alright we haven't taken off commission but given your example and the % I have achieved we are now well in profit. And these figures are for a base book of $100. I regularly make books for $1000 or more and when Black Caviar was running around I was holding in excess of $15000....and had no losers.