Ran this on my db.
The problem is a $10 chance on the tote could be $20+ on betfair.

It showed a negative expectation. The betfair odds in my db showed a difference of tote to betfair prices of 46% on these selections.

Taking your figures and applying the 46% differences I got :

Selections: 2549
Return: 1668.80 *1.46 = $2436
POT: -5%

After applying the 5% commission = break even or close enough.

Of course depends on the prices obtained. I am just doing some simple analysis here to give a quick indication of profitability.

My calculations show you will be laying on average a -0.95% expectation and any negative expecation is not a good thing. Expectation is worked out on strike rate and avg lay odds.