As far as I know, most broadband suppliers are still 'unlimited.'
You didn't answer the question about where you are located; nonetheless, look for 'dry loop DSL' aka 'naked DSL.' This is DSL broadband across the phone system but you don't need to have telephone service.
Typical prices are $30 - $50/month for unlimited usage at speeds of 1.5 - 7.0 Mbps.
To put 40 MB into perspective (and bear in mind that you 'ran out' in 4 - 5 days), my XP SP3 down load was ≈65 MB. Admittedly, that's unusual, but it doesn't seem unusual to download 2 - 15 MB/month just from Windows Update - for one computer.
google "Sprint mobile broadband reviews" what comes up is mostly positive, even for the 40 mb plan. i don't get it.
This service is not for use at home, but, rather, for the business traveller who can check email, search Google maps, etc., while on the road several days a month.