Where one finds the big difference between TABs & Betfair is when one is doing mutii-bets such as Dutching.

Example
Dutch 4 horses to a Book percentage of say 70%
Commission paid on profit.

Try doing the same with say a TAB
You wont get anywhere near this percentage on the same runners.
The commission is built into the price whether you win or lose on all runners , not just the ones that win , you pay through the reduced price in the form of their Over round on Book percentage which is usually 118.5% which includes their other trick of rounding down of divs, which no one likes talking about.

Example of Rounding down
e.g. If calculated divis actually 1.99 they round down to 1.90 they just keep the difference - funny that.

What about the 1.00 divs one often sees - that's funny too.
No one wants to talk about that either.

I would not want to do business with any one who blatantly snips punters in this manner, but they see nothing wrong with it - funny that.

I now know what the TAB means by chancing your luck.
Lucky to just get your money back after risking a ...
100% Liability .
Possible Zero div if successful, but you wont know until after the race, because what you see before the race, is not necessarily what you get.
100% loss if unsuccessful.

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CONVERTING BF PRICE

If one wishes to convert the Betfair price to a net price with com taken into account.
Then take 5% com off its Fractional odds, (not decimal odds.)
mutiply fractional odds say 4/1 x .95 = 3.8/1

Then convert to Decimal odds = 4.80


Example
5.00 horse.
4/1 x .95 = 3.80 + 1.00 = 4.80 Net BF price

Now this can be compared to TAB price.

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Cheers.