At first, when the site was launched, everything, from the messsage listener to the website, including the workers (a.k.a. the number cruncher) was running on a single server. Those were simpler times. Hosting came up to $10 a month.
Scars of War
Hex PvP Tools didn't have much traffic when Scars of War arrived. It really became popular when the decklists for the March 2017 Cosmic Crown Showdown were published, which increased traffic by a factor of 20 over a couple of weeks. The flimsy server started crumbling under the load and I had to separate the message listeners from the main site. Having dedicated servers taking care of the messages improved reliability and performance. The main server was still taking care of the site, the database and the workers, so I had to upgrade the hardware too. This tripled the hosting costs to $30 a month.
Things were going well and stable for a couple of months, but as more features were introduced, new ways to crunch the numbers were added on top of what was already there, and it became clear that having a single server take care of pretty much everything was not going to cut it, so specialized servers were deployed to spread the load and allow for better scaling. Hosting is up to $80 a month.
Dead of Winter
During the release celebration of Dead of Winter, which lasted for over a week, the server activity was so high, with all the gauntlet runs and ladder matches and auction house activity that messages were coming in faster than the server could process them. After a couple of days, the workers were over 12 hours late in processing the messages, so a new worker with extra CPUs was added temporarily in order to catch up. All these new messages filled up the hard drive so storage had to be upgraded.