It will always be cheaper to do it yourself. It will be cheaper to drop roadside assistance altogether and fend for yourself while on the highway. In fact you could rely on your own ability and cell phone to bail yourself out of any situation.
Or you could pay for roadside assistance and rely on them to get the job done.
Yes roadside assistance will be more expensive that is a given. It is easy to complain about a price once you have located the tire and installer locally.
However finding the right installer will not always be that easy.
Sometimes you call a plumber and he comes out and charges $150.00 just to tighten a loose nut and you feel ripped off, and sometimes you pay $150.00 to cut out and replace a bad section of pipe and you feel you got your money's worth.
Finding a tire on the road is always a **** shoot. Each situation will be different. In this case you really did not need GS assistance at all, that will not always be the case.
In the big picture $$$ value is not the overall deciding factor, getting back on the road in a timely manner will always be paramount. Unless you carry your own spare finding a tire and its cost will always be an issue.