Servers are likely the largest scaling cost you’ll have for a while. And, the amount of servers you need are primarily based on the number of concurrent users you have at peak hours. Having a database of 100k users doesn’t cost more, but having 5k users using at it once would require a fair amount of server capacity. And, the amount really depends on how server intensive their actions are. If they’re just looking at standard pages on your site that are the same for everyone (so they can be cached) then it’s not too expensive. If they’re all loading large lists of RGs that are personalized to them, then it’s a lot more server intensive.
You may also spend money on 3rd parties, like sendgrid, mailchimp, mixpanel, etc. Different tools use different pricing models, but generally the more usage the higher the cost.