I can see two scenarios where they might increase taxes.
First, many communities are suggesting that roundabouts *replace* existing intersections. In other words, they want to take a serviceable, but perhaps somewhat more dangerous, intersection and replace it, which obviously entails spending money you otherwise might not have spent.
Second, I strongly suspect the initial installation costs on a roundabout are higher than just a standard four-way stop. There is more pavement, more complicated grading (you don't see many slanted roundabouts), and a slightly larger footprint (meaning more land to acquire, especially relevant in populated areas) compared to a four-way stop.
I'm generally opposed to replacing busy lighted intersections with roundabouts, especially ones with multiple lanes of traffic on each road, as has occurred in some populated areas, like in Towson, Maryland. However, a simple intersection of two two-lane roads in a quiet area is an excellent spot for a roundabout, particular near highway off-ramps. I've seen these well-executed in many areas, including in Maryland.
I'd argue that replacing a serviceable interchange for no reason other than to add a roundabout would be a waste of money, with the offset of improvements in car safety technology we've had over the last few decades, but that it might make good financial and safety sense if the road already requires work.
