If you go in knowing how much you're spending (for the reseller account, billing solution, advertising, everything), you'll have an idea of how much you'll need to make to be profitable.

When I started off, I had minimal costs, so I just figured out how much the accounts were costing me, calculated in a percentage for time to set up, and so forth. Otherwise, you're just kind of flying blind. You might end up pricing your plans so that you make sales, but aren't really profitable. Your business plan will end up being "as long as I'm bringing in money, I'm doing okay"...which is not a very good business plan.