I could do the math and figure out my per soft cost on my last two homes we built but they were both 7-12 years ago...so not really applicable to today's prices.
The problem you'll run in to is a lot of builders won't build the plan you got of the internet...or even the plan you have an architect draw up for you. Most builders will only build the home plans they offer, even if they call themselves a "custom" home builder. The second problem is finding a lot to put that custom home on. Quite often, again, the builders will only sell their lots to people building their homes. This is often driven by city approvals for their permits, sometimes not. Getting around those hurdles, here's some other things I did in my current home:
Extra power outlets (It's expensive and timely to run these in the future...even doing them your self)
Data every where (I rant his myself once they had framed the house. I pulled two cat 6 data cables and a coax cable to every box. I put a box on every wall in every room in my house. I have around 4000' ft of cat 6 cable in my house
The electrician that was doing the electrical work on my house said it would have cost me closet to $10K to have that much ran if he was doing it...it cost me a couple hundred bucks and a Saturday afternoon of my time)
Conduit from attic to basement (My house is a multistory house, so this makes it easy if I ever need to get anything from the basement to the attic in the future. Such as wiring runs, gas lines, optic runs, etc)
Upgraded all vinyl flooring to tile (something you say can easily do your self in the future, but you rarely end up doing it
)
Ran 220 (To the garage for obvious reasons, also to the backyard for a hot tub. It cost us very little to do it at the time of building vs. doing it post construction)
Upgrade your carpet and padding as much as you can (Again, one of those things you can easily do in the future, but usually end up living with so-so carpet and sucky padding)
Nickel and dime things:
Window coverings (blinds and drapes)
Yard
Basement
It's been a while and I can't remember what else...we planned $10k for extra stuff for AFTER we were moved in and blew through it like it was nothing.
This was our second house we'd built, we wanted to do this one exactly how we wanted and ended up spending more than $80K on upgrades. Don't get too caught up the base price of the home, some builders include TONs of upgrades and have a high base price, while others include nothing and have a low base price. It was about location for us, so that limited our search greatly on which builder to use. For this second home we ended up using Perry Homes and were happy with them. They also have kept the integrity of our neighborhood exactly how they said they would. Our first builder was supposed to be the only builder in that neighborhood. When they couldn't sell the lots, they ended up bringing in another sub-rate builder who promptly put a surf blue home (at the time) out our front window. It was actually a couple streets away so was later blocked by other homes that went in...still annoying though.