Coinbase has a much larger limit if you verify yourself. For instance, on Coinbase I have $5000/week limit. And I belive you can get up to $1M limit with enough paperwork.
Bitfinex offers a .1% fee or $20 (whichever is higher) so for $3300 that would be .6% fee with the $20 minimum. For $300 though, that's 6.6%
I'm not sure where you get $11.51 for $300 at CB. I just attempted to buy $300 and it was only going to charge me $304.47, which is a 1.49% fee. There's the same 1.49% on a $5000 purchase. They do have a minimum, which appears to be $0.99 < $50, $1.99 < $100, $2.99 < $202, and after that it's 1.49%
EDIT: Oh, I see.. You're talking AUD, not USD. Not sure what the rate is there. FYI, in the future it's best to specify currency symbol if you're talking about Dollars and not USD.
According to this page:
3.99% is the base, but in the US they wave some fees depending on the payment type. It appears that in AU they only allow Credit Card purchases, and not wire or EFT. That kind of sucks for you.