Everything below is about the official MMO world only. The single-player mode and unofficial private servers are available without subscribing.
This makes Screeps more like a hosting or cloud computing service rather than a traditional online game. We offer you server CPU time and fixed amount of memory that you can use as you wish (though it’s still preferable to use it in relation with the game!). Though a new player gets a quite low initial CPU limit, after upgrading your game level you can end up with your limit up to 300 ms per game tick which is quite a serious hosting asset.
As we designed the game, we were considering numerous options and finally came to a conclusion that justification of such expenses requires only one model: game time (or, in our case, CPU time) subscription. And note that we don’t offer multiple subscription options: the subscription cost is the same for all players. Thus we avoid creating game advantages for certain groups of players since all are provided with equal opportunities to use server resources.
If you don’t wish to use such resources on general terms, you can just confine yourself to purchasing a starter kit on Steam. A minimum 20 CPU amount will be reserved for you on the server, but it won’t grow following your game level. That's a perfectly fine way to play Screeps too, since we have a standalone non-subscription world shard, where you can compete with other non-subscription players on equal terms.