All runners:

Tatts average price : $1.67
Betfair Average price : $1.87 (12% higher)

For winners:
Tatts average price :$1.65
Betfair Average price : 1.83 (11% higher)

The largeincrease is removal of commission (5%-6.5%) and moving from the bet side of the screen to the lay side of the screen.

So whn you look at these figures your 10% is not a good estimate. You need to take at least 12% and the above fiogures do not include commission. You need to work out commission for your betting technique as its a lot harder to work it out laying but I believe it would add at least another few cents.