Thanks for your replies Shaun and Wesmip1 but I obviously haven't clearly explained my dilemma.
I created my system based on horses which in the past paid =>$25 on S-Tab. The fact that I could get $100 on Betfair is irrelevant if today it is only showing say $15 on S-Tab because then it is not a selection. I'm afraid I get dizzy just thinking about the huge number of bets involved if I were to extend the criteria to a horse starting at =>$25 on any TAB or bookie or exchange, not to mention having to go back and retest on all those markets.
I have noticed that the longer priced horses on S-Tab do tend to drift after betting closes but I don't have any stats as yet to confirm this or any sense of the percentages, just gut feeling. If that turns out to be the case after further monitoring maybe I can include those at say, $20 or maybe $22 and hope that I catch those that drift and not too many that shorten.
Cheers
Ron