IME the most savings on any job is in Labor, if you can do things yourself acting as GC, you can save quite a bit.
One big thing is daily clean up, at the end of each working day on one of my projects I or my kids went around doing all the clean up, wood scraps, bits of wire etc...
There is no way I am willing to pay someone $20+ and hour to sweep.
As a GC for one job the others are correct in that finding the best subs is an issue. however if you have the time then it can be done. I used to have a great electrician who did side work on the weekends, so I would get him in there when he was available, he also had no issue with me putting in all the boxes, drilling holes, pulling wire and he would do all the terminations. over the course of a job this saved me thousands.
This worked for me because I was on site most days anyway to solve issues with other subs.
Another spot I saved $$ was drywall, I would hang it all and just pay the tapers to do the finish work.
One last thing is that you can save a lot by sourcing materials yourself, where a GC will order all materials from one place you can use your time to find the best prices, of course to do this you have to know what you need and when you will need it. Wire is a good example, knowing that you will need say 2000' of 12-2 you can watch sales and pick it up in advance and have it ready for the elec.
I have a friend who built his entire home from materials from RR auctions, yes he had to buy entire car loads of damaged wood, windows, etc. but he paid pennies on the dollar for them and had a nice big pole barn to store them in.
In the end after selling the extras he built his 3000+sf home for under $25k