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.

They have ~2500 emails, no socials and seemingly only a few backlinks to startup listing websites.

That does not seem worth 10k to me. What do you think?

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!

for how much?

You’d be amazed at how much projects with a little more effort put into them go for then! There’s a level of companies with niche products and potentially no revenue being sold for tens of millions. Granted this is partly aqui-hire but 10k is very little money in the grand scheme.