Database Costs?

Pretty Cheap. It depends on how many pieces of data your storing for the user, the data types, and the type of database you pick etc. I recently worked on a site with almost 100,000 users that had a decent amount of user meta and posts etc and the db size was only 1.2 GB. A standard $5 VPS should give you 10-30gb which is way more than the database needs. If the site has a lot of reading and writing to the database you will need a slightly more expensive server that has more memory and CPU cores. If the site has photo uploads then that we can guesstimate it will take a mb per photo which would be 10gb ( 10,000 x 1mb = 10,000mb which is about 10gb ). You can run tools like pngquant on a cron job to shrink the image sizes. I am going to guess you will need to spend $10-$30 depending on the details. If a large amount of users are all logged in at the same time which is rare for most sites then you will need way more CPU/Memory.