Too many factors! Depends on how many domains you sell and what your profit margin is, vs the sitesโ earnings.
If you get a site to the point where it earns reliable income, itโs a gift that keeps on giving. Donโt think of it as โdeveloping a domainโ but as โbuilding a business.โ
But you need to put in some effort to make it work, and some people donโt want to be bothered.