Virgin Mobile USA has just increased data allocations for Broadband2Go users.
$10 gets 100MB for up to 7 days
$20 gets 300MB for up to 30 days
$40 gets 1GB for up to 30 days
$60 gets 5GB for up to 30 days
Source:
Mobile Broadband Service | Virgin Mobile. Note this is a CDMA carrier and you will need to get one of their dongles.