Yep. A matched set of broadside mount installations (everything but the gun, basically) is about a half to a third of a spinal mount installation. Part of the reason for this cost is that broadside mounts get a lot of use out of the existing framework. To get an idea of how this looks, have a picture or two.
The thing that makes spinal mounts so expensive is that they go over that very large framework, while broadside mounts go in it. The difference adds up fast in terms of weight, material, complexity, and therefore time spent which has direct influence on cost.