IF(market_mbnum=0,default_backa,IF(market_mbnum=1,default_backa+(bm_backa*.40),default_backa+(bm_backa*.10)))
mcbee,
I copied and pasted your latest formula and tested it.
Here are the results for one race
     BET     BELOW ARE WHAT THE BETS SHOULD BE
A   5.00    5.00
B   5.00    7.00  previous bet *1.4 (2nd bet always 40% higher)
A   5.50    7.70  previous bet *1.1
C   5.00    8.47 previous bet *1.1
D   5.00    9.32 previous bet *1.1
E   5.00   10.25 previous bet *1.1
C   5.50   11.27 previous bet *1.1
E   5.50    12.40 previous bet *1.1
C   5.55    13.64 previous bet *1.1
The procedure should be.
base bet     5.00
2nd bet       7.00  (very first bet *1.4)
3rd bet        7.70  previous bet *1.1
4th bet        8.47 previous bet *1.1
etc..... all bets after second bet is 10% higher than the previous bet.
horse ID does not come into it. therefore horse names or ID's need not to be remembered.
The only criteria required to trigger a bet is that there is a change of fav order
and the name of the horse does not matter. The first bet is a base bet. The second bet is 40% higher than the first bet and all other bets thereafter are 10% higher than the previous bet. A horse could be bet several times but each time treated as any other horse and betsize it 10% higher than previous bet.
Sorry for all the repetitiveness but I want to explain clearly as possible. 
Thanks if you can help further
R