Triffid Park prices for most plants are very reasonable. You are right about the price of the Nep's. A cheaper option for the Nep's if you are happy to import would be Captive Exotics. I get alot of my Nep's from here as they tend to be a little cheaper and they have a larger range. The plants here range from $10 to $50 Aus.
Here is a link to that site.
http://www.captiveexotics.com/
Currently $1 Aus = 2.35 MR