Figure out your costs…how much will it cost to develop? How much will it cost to run? Do the operational costs increase or decrease over time? Do they increase or decrease as you add more customers? How much will maintenance cost over time? How much will adding new features cost over time?
Don't forget the vig that the app stores and payment processors take (anywhere from 2-5% depending on the whims and fancies of the companies involved as well as the credit card the consumer used, if you take credit cards).
Don't forget compliance costs, e.g. if your service has to access another service's data in the cloud they may demand SOC 2 certification or some other certification.
No, you're not going to be able to accurately identify each and every cent / yen / euro of cost but you need to identify as much as possible and then set your price as some multiple so that you can eat moderate fluctuations in costs or revenue or items you missed and hopefully have a decent profit.
I think too many SaaS's underestimate the basket of costs involved in providing their probably perfect useful, viable service and attract a decent customer set (or they use investor funds to subsidize the cost of service). Then when they actually have a sense of how much money they're losing per month or per transaction they radically change the cost structure with too short a timeframe for customers to adapt. The larger the customer, the longer it will take to absorb a change to expenses. Doesn’t matter whether they can afford it or not, what matters is how the increased usage fee intersects with the customers annual or biannual budgeting process.
Just my opinion, I mostly walk a dog now.