The first approach would be to search for topics here that deal with selling software. There are not so many of them.
Summed up, here it is.
Are you selling a licence, your coding service or full app with all code and resources?
You have to see the price of similar software and then set your accordingly. Or you can ask yourself "if I want to return invested money/time in 100 licences, the price is...". Then see if the price is too high/low and fix it accordingly.
You have been hired to do the coding. Calculate your hourly rate and multiply it with 80 work hours you spent on the project.
Selling all code.
This is the hardest thing and there are multiple topics covering this. Basically, you have to calculate all costs involved creating the software then multiply it with something. This "something" can be either time in which the buyer will return money or some other formula. There are many of them on the net. The time in which the buyer will return invested money is time needed to sell so many copies of software to return money the buyer spent buying the software. If this time is 1 year, for example, then try to estimate how many copies he can sell in 1 year, then multiply it with the price of 1 licence + add your developing costs - the price you get will be close to the real price.
Since this is the market, there may not be real formula to calculate the price of software. Many will tell you to get as much as you can, regardless of all formulas results.