Betfair is $5, IAS usually have a $1 minimum but sometimes increase that during high traffic days ie. Spring Carnival. Other bookmakers I am not sure of.

I am just back on the punt after a hiatus for about 10 years, got married had kids etc. The options available today are mind-boggling. In the old days, you either took the tote odds, or went to the track when you would compare odds and the tote in the 35 minutes before a race. These days you have a lot more time and can get some huge value. I had $9 on Swiss Ace @ $65 in the Oakleigh Plate. Tote paid $37.60 and SP was $31.