I posted this in another thread yesterday. It may be too much of a hassle for some, but may be worth looking at:
My current UK carrier is way too expensive when roaming in the US, so when I'm in the US, I use a "3" PAYG (burner) sim with the "feel at home" option. This gives me 12GB of data per month in the US for 30 GBP (approx 40 USD). As it's a PAYG, I only pay the 30 GBP for the months I'm actually in the US and let it lapse the rest of the year.
If you've any friends/family in the UK, you might want to see if they can get one for you and send it over by post.
I'd check the small-print though - AFAIK it does need to be activated in the UK first, and may stop working at some point if it's not used in the UK every now and then.
Edit: This is the product I use:
http://www.three.co.uk/feel-at-home