Cs-cart mod: decimal quantity ordering (1817944)

My client needs a modification to the way cs-cart works to allow his customers to order decimal quantities for weight based items. i.e. 0.2 would be 200 grams of the product when priced per kg. The cart will have both normal priced items and weight base items. Ideally the mod needs to be written so we can still upgrade the cart when required. Budget is $100.