Usually, the price is for the email list, backlinks to the domain name, various social accounts with followers, etc. The price for the actual app is a small part of the equation.
if it's making $350/month, that's $12.6k for 3 years. I think a general rule of thumb for website valuations is 3 years of ARR so seems about right. That's assuming that the website will never grow and will never has any other possible income source.
Never grow? It will shrink. The buyer got robbed. Churn is one the numbers investors watch like a hawk in a SaaS, and I can’t imagine this service retaining more than 30% of their users over several months.
My side-project only had ~500 email subscribers, no MRR, but pretty good domain authority because it marketed itself.
And still it got acquired only after 3 months!