Next G has always been marketed as a premium product, so that price (which applies to any plan, including prepaid) isn’t surprising. The only effective way to skip the cost is if you sign up as a new customer to a long-term contract. For the moment, the product is Windows only, but Telstra is promising Mac software by May.
The best selling point for Next G is its near-universal coverage, but if you need that, the high-speed option may not be your best bet, as the enhanced speed (normally nearer to 8Mbps than 21Mbps, by all accounts) is only available in “selected CBD, metro and regional areas”. If you decide to give it a burl, share the experience in the comments.