The Missouri DOT recently released a long range transportation plan that included this little table:
I'm guessing that MoDOT's approach to roundabouts is similar to the pimped out roundabout we saw in Wisconsin. That's the only way that I can explain how it would cost so much more to build something so simple, especially when compared to a traffic signal.
Roundabouts provide great traffic flow while dramatically reducing accidents and nearly eliminating fatalities. We should be tearing out traffic signals around the country and putting these in right now.