It’s land acquisition, drainage, utilities. Costs we can get down for sure. But idk how out of line we are with France, which does them well. Apparently the one in the pic was €433k (2014). Quoted average is 2011 I think. (Would appreciate being corrected on this, I can’t really read French)

www.lanouvellerepublique.fr/vienne/ces-g...www.ouest-france.fr/pays-de-la-l...

Ces giratoires qui font tourner le monde

Pour aller à gauche, tourner à droite

You can build them just about as cheaply as you like. But you sacrifice stuff that matters. See the pic, where space was a constraint, and exit speed across the crosswalk is way too high. Or you can leave out the splitter islands that protect pedestrians. Or use stick-on/moveable center islands, etc