I'm after a trigger  for horse/greyhound racing that will place a fixed liability LAY SP bet of $Var?, bet #Var? secs after official start time, if total matched amount exceeds $Var?, last matched bet is >= $Var? which is dependent on number of actual starters #Var? (5 - 16). 
Var? means a user defined amount.
I guess you will provide a cost up front.
Cheers