The one way to do it is to multiply the hourly rate of the developer and the number of hours that the work took from him. Of course, it depends on the rate and on the complexity of the project, since complex projects take longer to complete, and therefore more money will be needed.
To this figure, you must also add the cost of hosting. All in all, according to our calculations the average cost of a SaaS app ranges between $60,000 and $100,000 (plus monthly maintenance fee).