A SaaS business is valuable because its subscriptions renew without anyone selling them again. Ask for MRR (monthly recurring revenue) month by month for the last 12–24 months, broken down into new customers, expansion, contraction and cancellations. Growing MRR spread across many small customers is sturdier than the same MRR from three large accounts.
Then look at churn: how many customers, and how much revenue, leave each month? Find out where new customers come from, what it costs to acquire one, and how many hours the founder spends on development, support and sales. If the seller is the only developer, their knowledge of the codebase is part of what you are buying.