The cheapest you can do it on my network which is probably the most 3G-orientated network here works out at 23 dollars a month for a 15GB allowance. This is comparable to a wired network in price but the download speeds can be much faster and the allowances are more or less uncapped on wired. I personally have an expensive PAYG option which is 23 dollars for 3GB a month but that works out about right for me most months data-wise so in reality I won't really gain anything financially enough tying myself to a contract...I don't watch high-drain movies or games.
Just checked your 3G prices and they are very expensive....strange seeing as we view the US as a cheap country generally. Here's 3UK's PAYG (the dearest option) prices against the US's monthly plan (cheapest option) price.
A 15GB monthly plan on 3UK is 23 dollars against 50 dollars for 5GB in the US...wow! That's three times more data for half the price.:0