Hi Mark,
I passed your lay system rules to a person who has a Betfair database going back 11 months and here are his comments,

There were 710 selections for 99 winners returning 530.83 The profit here is approximately 144 units.
This had no odds restriction. It was in profit 9 of 11 months tested. The months when it was down it hit long shots.
Restricting this to maximum $20 lay price reduced it to 513 selections for 97 winners and a profit of 26.

I don't see this being a system I would bet. Most of the profit comes from the longer then $20 priced horses. Based on this 1 or 2 winners in this price range could easily wipe out this profit and even make it show a loss.
Those under $20 are not up by much either. 2 winners above $10 and you have wiped out the majority of your profit.

As you can see he has proved that your lay system did perform over the 11 months testing using actual lay odds and after deducting the Betfair commission.