What about making a game fully moddable? How does that play into sales?
Are we talking LittleBigPlanet customizable? Or Counterstrike out of Halflife modding?
If you mean the former, custimization is all the rage of course. If the game is a social game, in that you can share your customizations, it becomes part of the gameplay, even if it's just dressing up your hero. It may or may not be worth the effort to get into the game though. I guess that would depend on how much time you feel you'll be implementing more peripheral aspects. I'd considering it 'polish' unless the game is based around the concept of customization.
If you are speaking about modding as the latter example, which I believe you are it should help -- provided you already have an audience. Not having any developer experience with this, only customer experience, if there isn't a strong community (for a specific game or developer), there isn't much of a point to modding. Who is going to be interested in doing the modding? I would think (again no developer experience with it) that re-releasing a project that did well, or releasing a sequel or expansion to a project that did well would be the best time to introduce modding capability. Once you have a game that people want to play, there will be some that will want to expand or change it. Don't assume that people are going to want to toy with your game before you even have players.
Good code (IMO) always keeps expandability in mind. I wouldn't put too much effort into mod-ability for your first commercial game though.