Well firstly the costing will be determined by what type of software you have/want; how much has it cost to develop and how your sales model forecasting is looking like for the next 2 year. How many clients are you planning to rollout to, as this will help determine when you will reach your break-even point and you can from there start calculating base cost. This you have to do before selling the product, otherwise you will over or under value your product quite severely. Look at what your competition is doing as a reference as well. Costings and selling prices are a complex process you have to go through before actually selling anything successfully. You will need to do some more research in order to answer these questions.
As for SARS, I'm not an expert on the subject, but you first have to value your company and your product before doing taxation processes. And to get the value of your company, you have to go back to the top part of my comment....
