Since you are adopting B2B eCommerce for the first time, it is justifiable that you probably won't have any current budget line detail. But if you manage to figure out the best B2B eCommerce platform, it goes without saying that a large number of your cost savings will be relatively straightforward.
The thing is that once you start taking advantage of these cost-related benefits, you can be ready to look at a subset of e-commerce solutions that take into account the reality of your budget.