It depends on the kind of customization and the overall scope of the game.
Some kinds of customization are lend themselves well to asset reuse and makes other types of content cheaper overall. For example, spending development resources making lots of different options for clothing can mean it costs less to create a lot of additional NPCs that are visually distinct, each wearing those variations on clothing that the player can obtain. A robust character creator can theoretically be used to create a wide variety of game NPCs as well.
Some kinds of customization add additional overhead and work. For example, allowing many different types of weapons can eat into your animation budget. Body size differences can cause camera and equipment clipping issues. There are other general tradeoffs - if everything is a variation of player-usable assets, it often comes at the cost of specific NPC uniqueness (i.e. it would be possible to make the player character an exact doppelganger of a plot-critical NPC).
That said, creating character customization content requires a notably different set of skills than cinematic design, level design, encounter design, combat design, item design, etc. The developers creating the customization options won’t be creating more levels or environments or cinematics. At a very high level, the overall team headcount will depend on the overall scope and design for the game itself. This determines big decisions like the number of levels, the overall story beats, the cast, the core gameplay, and so on. This also dictates the number of texture artists, animators, engineers, designers of various types, and so on the team will eventually grow into. This means that, if customization is a big part of the game like it is for Saints Row, the budget for customization will be larger than a game without much customization.
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