I just had a quick look at some of the hosts out there, and the bandwidth figures vary hugely.
0catch.com - 100Mb space and 600Mb bandwith with 1Mb file size limit
Fortune City - 25Mb and 3Gb
FreeServers - 12Mb and 0.5Gb with 256Kb max file size
Netfirms - 25Mb and 1Gb
Spaceports - 20Mb (can be upgraded for free) with 1Gb (5Gb if using paid ad removal)
Tripod - 20Mb and 1Gb
If you're wondering, I picked those by first checking hosts that had been in business a while, included ads and who actually stated the bandwidth in an easy to find place (many bury that info).
I don't know about the other hosts, but Spaceports has been losing money for several years now, and are subsidised by their parent company (who offer paid hosting).
I suspect many of the others also depend on the paid hosting options, considering the low rates paid for banner ads these days.
If you do decide to raise your bandwidth, I would suggest that you add a file size limit if you don't already have one.
Those with sites that mostly consist of HTML pages can get by on very little bandwidth.
For example, I have a help site that gets something like 11,000 unique visitors/70,000 page views a month and use maybe 600Mb of bandwidth. If I didn't have WS_FTP LE available as a download, I'd use even less.
From your point of view, allowing users more space is more cost effective, as bandwidth is a constant drain on funds, whereas hard drive space is only paid for once (per account).
If you raise your bandwidth level too high, you're going to get a higher percentage of abusers I'm afraid. If you're just starting out, you might want to limit the number of new users by keeping your specs low, until you have a clearer idea of whether the service will be able to pay for itself.
You can always offer additional free features to members who have followed the rules for a certain length of time.