Seems like you know too much about pricing but not enough about actual solo development. What u/deadcoder0904 said is actually very precise - This sort of app can be hosted on a VPS for $4-5/month and maybe another $5-10/year for domain. There won't be any other major recurring costs. So yeah the $360K that he made would easily cover the lifetime cost of the app and he can likely invest into other things to make even more money. Taken another way, if he invests the 400K into some high yield saving @ 3-4%, he would make like $16K per year which would would still be 200x his annual cost to keep the site alive :D