Congratulations on having something to sell
My own view is that you should not sell anything outright. A good formula is to sell at £x per user and then to charge n% of £x times the number of users per annum as maintenance. The maintenance covers the cost of bug fixes, enhancements and perhaps some email support i.e. the customer base get all these for free, and the maintenance goes to pay for your bread on the table
It is difficult to quantify £x or n%--you are probably better placed to do so.