Shaun, I have read your question, but somewhere along the line it boggles me a bit.

Average price required. OK
System selects same horse, what is the average price ?

Do you mean, unit allocation amount, to bet on all the runners concurrently, and if system A and system B had the same selection ??

Or,
A & B same selected runner was paying 5.00, average price of the 5.00 ?

??