I couldn't help myself and had to do a real example.

Canterbury Race 5 today. I hade to use tote odds (tatts) as I can't find any bookie prices anywhere.

Tatts Odds :
2 - 4.40
4 - 1.80
5 - 2.10
6 - 3.80
7 - 2.50
8 - 3.90
9 - 4.90

Betfair odds from the betfair advantage tool (lay odds as I assume your laying)

2 - 3.30
4 - 2.04
5 - 1.54
6 - 2.46
7 - 2.00
8 - 3
9 - 4.10

Ok lets assume you backed the entire field with 10 units each.

I assume you layed the field for $10 each runner.

Result was 4,8,7

Lets do it horse by horse.

2 - Breakeven.
4 - Gain of $8 from tatts and loss of $10.40 on betfair
5 - Breakeven
6 - Breakeven
7 - Gain of $15 on tatts and loss of $10 on betfair.
8 - Loss of $10 and loss of $20 on betfair.
9 - Breakeven

I'll adjust for commission so betfair gain of $40 - 10.40 -10 - 20 = a loss of 40c.

Tatts Gain = 8+15-10-40 = -27

In the case above you have made a loss of 27 units. Are my maths wrong ?

Here is another one so lets see if it profits. Morphetville R2:

In this case I'll assume you get best of the three totes for place odds:

2 - 11
3 - 2.1
4 - 3
6 - 1.9
7 - 9.1
8 - 2

Layed at :

2 - 9.6
3 - 1.6
4 - 1.87
6 - 1.2
7 - 9
8 - 1.48

Result was 6-4-3

2 - Breakeven
3 - Loss of $10 and loss of $6.00 on betfair
4 - Profit of $20 and loss of $8.70 on betfair
6 - Profit of $9 and loss of $2 on betfair
7 - Breakeven
8 - Breakeven

Result for betfair is 30-6-8.70-2 = 13.30 * 0.95 = 12.63
Result for Totes is 20 + 9 -40 = Loss of $11

This means you made $1.63 and that was on the best available odds. Lets use NSW divs instead as they seemed to be the higher few opn avg:

2 - 11
3 - 2.1
4 - 2.90
6 - 1.4
7 - 9.1
8 - 1.5

Based on these odds :

2 - Breakeven
3 - Loss of $10 and loss of $6.00 on betfair
4 - Profit of $19 and loss of $8.70 on betfair
6 - Profit of $4 and loss of $2 on betfair
7 - Breakeven
8 - Breakeven

Result for betfair same at : 12.63
Result for NSW is -40 + 19 + 4 = -17

Its going to be a hard slog to get the equivalent of best tote odds for placings with most bookies on all meetings meaning your very restricted and even then you might just edge a profit but is it worth the risk ?