Cost is very dependent on location and what you want in the house.
But, there is no doubt that building is far expensive than simply buying a new existing house and far more expensive than an older home.
We actually bought land twice intending to build. We works on plans and spent money to get a design and both times I ended up bailing and selling the land.
Reasons we decided not to build:
1. Cost. In the second go round we were working with a builder who would custom design the home. I looked at the cost for the features we wanted. It became clear that once we added in the land, the design, etc. we would be spending over $100k more than buying a house. It was going to cost about $400k and that was making compromises on some things where I wasn't getting exactly what I wanted. (This was several years ago). While still working on the design, I started looking at existing houses for sale. I found the house we ended up buying. It overall had nicer finishes than we were going to have in the house we were building. It was a little larger. It was 5 years ago and in great condition. We paid $295,000 and sold the land and didn't build.
Do I regret it? Not really. I mean, yes, there are times that I yearn to have the exact layout that I want. I actually agree with you about living and dining rooms. Currently our dining room is an exercise room. The living room is unused 95% of the time. When we sell this house (which we will do most likely for reasons not related to the house itself), we will probably buy a smaller house and either DH or I will use the living room as an office.
There is one part of this house I don't like (master bath layout) and we originally planned to gut it and re do it. But, since we will likely sell we probably aren't going to do that.
2. The other reason for cold feet on building is knowing everything that can go wrong. I have seen horror stories in building, both financially and problems in construction. So, I am very leery of problems with building. However, I would probably research a builder carefully and chance it if it wasn't for the cost factor which I just haven't been able to get over.
Alternatives to building include to keep looking for the ideal house, buying a house that has part of what you want and remodeling to get the rest and buying a spec home under construction where a lot of work has been done and you can still make changes you want (an alternative is to buy in a subdivision where the house is not built yet but has a layout you like and you can pick out the finishes).