Don't buy branded aquariums. They are expensive.
Usually I buy a China made brand. They are the cheapest.
They will cost less than NZ $25 for a 6-7 gallon tank or NZ$18 for a 5 gallon tank at my place.
But I guess at your place, it will be more expensive.
Here are some examples:
For Bettas prices, each country prices will be different.
In my country, you can have $1 to $40-$50 or even more for each Betta depending on the species and size.
The species will affect the prices the most.
But based on NZ online stores(which they called it Fighters fish), I think it will cost you between $20-$40 for those with nice fins/tail.
But they don't seem to show the normal type of Bettas which cost less than $5-10.