Despite the cost, Bithell says he cut corners with Volume. For example, the main character wears a mask. Right there you don't have to worry about facial animations or lip-syncing. The environments are abstract meaning the team can use fewer textures, and have an ex-PS1 artist design most of the levels. He calls this the "Kevin Smith approach."
"All he had to do for Clerks was throw in a single line to explain why the shutter was down, and he could film at night savings about a million on what a proper set would cost. Every choice we've made has been made to make the game we want to, and cut around the expensive stuff.