I have 6 months data but it confirms over $30 on betfair is suicide.

Laying all selections over $30 ended up with 23034 selections and if you layed at the available odds you would have paid out 25992. Approx a 10% loss before any commission is taken into account.

Over $100 showed 5773 selections and you would have paid out 6431 before commission is taken into account.

Laying the longest priced runner in the race (min $30 price) showed 7181 selections (or races) and a payout of 8840 before commission is taken into account.

I played around though and found a profitable method for you if you can stomach it and have a deep bank.

You can use unitab ratings or cp ratings. cp is available at r and s neurals.

Stick to those which rank at least 14th or worst in each of these and stick to laying horses above $80.

This had 791 lays for 1 winner at $90.

Obviously one winner could make it bad though so I wouldn't be doing it.