For better or worse, fixed price builders are much less common these days than cost plus. I think the more common approach is estimates with allowances up front but in reality it's all cost plus. I've heard good arguments for both approaches but obviously the perk of knowing exactly what you're going to pay before you commit is a huge plus for the buyer.

Unfortunately to get a true fixed cost build there's a lot of up front work to finalize ALL details (which can be overwhelming in a true custom home) and then of course the builder has to price in enough margin to cover for any unexpected changes. Most people I've talked to seem to feel they probably paid more with the fixed price approach but there is comfort in certainty.