Pick up a few $2 virgin pre paid sims and activate them as wireless broadband. This will give u a minimum of 300mb per sim and there are 2 sims in the $2 packet. So it equates to $3.33/GB. They also work for calls and sms. They can be recharged same as normal prepaid sims and perfect for the n900 because they use optus's network which is 2100mhz and 900mhz.

There is a few threads on whirlpool about them.

3 is only good in built up areas but charge ridiculous amounts for data and calls when you go outside their own towers.