The least expensive and easiest due to building codes is obviously to build in an existing structure, such as a basement or second floor, or maybe an unfinished attic. That way, the outer structure is already done.
We're looking at building a garage the same size as the one we have, 26x28. We haven't gotten any bids yet because we're focused on the house interior ATM, but I estimate about $40K for a garage including concrete pad, lumber kit, a couple extra garage-quality windows, labor, and wiring. Finishing the inside with insulation and OSB or Sheetrock would cost a bunch more. My estimate includes no DIY, secondhand materials, etc, and is based on other projects we've had done and talking to inspectors and contractors lately. I always figured the cost of materials, then multiplied by 3 to get cost of materials + labor. It always came out pretty close.
Of course, the more you can DIY, the more you can cut costs. Same if you're a good scrounge like I am, have a knack for finding bargains, seek out discounts and secondhand sources, and aren't afraid to clean and paint things like cabinets. Also if you can flex on your design because something nice but not your first choice will save money.
Some things are nonnegotiable, like code requirements for things like wiring and plumbing. In those cases, there's nothing to do but shut up and pay the pros. We're going through that right now.
If you're trying to figure costs down to the penny, don't. Have a healthy cushion for cost overruns, because there will always be some. Building supply prices seem volatile right now. I was truly shocked to see the cost of the windows we bought 2 months ago had jumped from the regular price then of $260 to $400 regular price now. If we were on a to-the-penny budget, we would be in big trouble now. Luckily we had already bought the 4 windows we needed in that size on sale for $230 two months ago, but that's just one example of how easy it is to end up unexpectedly needing more money.
It's fun to think about the end result, but stuff like furniture, decor, and paint color should be the least of it, IMO. The most important things to consider are the boring things, the things we don't even notice but use every day and take for granted, like wiring and plumbing. That's where the biggest, least flexible costs usually are.