IMO, a custom home gets a custom AV system. Having said that, it's easy to put a set price on a package that might include, say, 4 rooms/zones of audio. We also start out with a basic security package that includes x door contacts & y motion detectors, which we add onto as needed. FWIW, all of the builders we work with do almost all custom homes, and probably 95% of the time we get in front of the homeowner before rough-in. Usually the builder gives a low-voltage budget, and anything beyond that is handled directly with the homeowner.