Erm... I think you'll have a problem coming up with a formula as disk space and bandwidth have a tenuous link at best and for the most part it is easier to count them as mutually exclusive.
For example, web site hosting A could have 100mb worth of content (i.e space used) but be an extremely popular web site hosting with millions of visitors a week, therefore using 1000's GBs of bandwidth a month.
Whereas web site hosting B, could have 10x as much content (ie space used) but have only just started up and not been popular at all and only use 300mb of bandwidth a month.
Your best bet, like you have done, is to see what others are offering, see what you can offer, see what you can give extra and what makes your package the one they'd go for over a competitors.
Most of the time this will go much further than simply space and bandwidth but go into areas such as is Fantastico offered? is PHP / scripting offered? What version? Am I allowed to hotlink on the server? etc etc etc.
You want to work out what the cost of your reseller package is and the proportianlly (based on the features of the package) divide your expenses up between the packages then add a bit on top for your profit.
Hope that helps a little - and if you have any more quesitons feel free to ask
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