First off I do not know this fish but did a bit of searching for what it is worth. As to price I don't seem to see any but $30.00 for 5 of just about anything is probably OK.
My concern is the tank size. First, from what I found, you need a minimum of 8-10 of these fish. It is sort of hard to really say as it would seem that there are many variations of this fish but, on an average, they seem to get 1.75 to 2.25 inches in size. If this is the case I can't see a 10 gallon tank being large enough.
If you can give the scientific name of the actual fish it may help narrow things down.
I assume that's Australian dollars? We need to be specific as many other countries use dollars and $30 in one currency will be different from $30 in another.
It depends on which river system they came from but it's about average. Personally you should have gone to your local creek and caught your own. Brisbane is right in the middle of rainbowfish territory and you have Pseudomugil signifer and Rhadinocentrus on your doorstep.
Contact ANGFA Queensland and go along to a few of their meetings. There are plenty of people there who are willing to share fish and eggs. They also have field trips and take members to local waterways to do collecting. Google ANGFA or see following links. If you meet an old guy called Adrian Tappin, talk to him because he is incredibly knowledgeable about rainbowfish and you could learn a great deal from him. It's his book in the second link below.