The space combat sim's website has updated with a new FAQ that outlines the game's projected system requirements (it recommends an i7 CPU and a GTX 670 or better graphics card, but keep in mind that launch day is two years away). The FAQ also mentions Star Citizen's modding philosophy and its payment model.
Modding is encouraged, as Roberts says that players are free to tweak the game however they wish on private servers. The devs also "hope to institute a mod approval process that will allow the best of the best player-created ships and other additions to be integrated into the central persistent world as well," according to the FAQ.
In terms of payments, you'll need to buy the game initially and that's basically it. There will be no monthly fee, though "some in-game items may be available as microtransactions." Unlike most free-to-play titles, Star Citizen's cash shop "will never sell anything that can't be acquired through honest (and fun!) gameplay."