Hi Benny,

Someone else will be able to answer this better than me but I'll give it a go.

Betfair have a minimum bet amount of $5 (back or lay) otherwise they have a minimum of $30 liability on the Starting Price (SP).

If you are laying under $7 then a $5 lay bet will give you a liability of $30 or less.
If the price is $7 a $5 lay bet will give you a liability of $30 ($7-1 = $6 x your $5 lay bet = $30 liability).
If the price is $4 a $5 lay bet will give you a liability of $15 ($4-1 = $3 x your $5 lay bet = $15 liability).

If you want a liability of $30 for any price horse (1.01 up to 1000) then the best way is to bet at the SP. Make sure the Betfair starting price box is ticked on the website (in the same area as the back & lay and market depth squares, click on the right hand side lay SP button and then enter your $30 liability where the place bets box comes up on the right hand side of the screen. If the SP square isn't there then you need to go to more options and tick on projected odds.

If your using it for win betting (backing or laying) and are placing more than 1 bet then the total liability for the race will still be $30.

I hope you have a big enough bank to be betting those amounts.