how much did it cost you to build, and how much do you want to add to offset and then secure a profit from that outlay (taking into account the fact the you will repeatedly sell the same software over an over). You could build discounted cash flow models and other spreadsheets to model the returns over time and use this analysis as a guide.
Find competitors or analogues of your offering in a similar/the same sector. Price yourself competitively against their rates.
What is the value that the client will attach to the solution. How much will it save them? What is the opportunity-cost of not procuring the solution/offering to the client. By modelling this and pricing your offering competitively against this number, you could make a sale that the client considers a good deal.
IMHO, Value-based pricing can often yield the best profitability, but you need to make sure you are pitching at the right level, and your patter is good.