In one hand, ICBMs used fancy, dangerous, poisonous chemicals as propellant. That must be more expensive as fuel, and might necessitate more expensive infrastructure than liquid hydrogen or kerosene. Plus the old problem: "a hammer from a hardware store is \$5; a hammer made by military contractor for the army is \$5000"
On the other hand, unlike with most civilian rockets, manufacture of ICBMs was streamlined. They were produced in thousands. That's a massive economy of scale.
Of course ICBMs being suborbital don't have a "natural" payload-to-orbit value. OTOH they carry quite heavy payloads (warheads), so either replacing these payloads with yet another stage, or even just reducing the payload, most of them could be used as space launch vehicles. The START-1 system is an example of such a conversion (but of course the actual cost is hidden in the fact military surplus rockets were used, which would have to be scrapped otherwise, so the launch costs doesn't really reflect the manufacturing costs).
I wonder how they compare cost-wise in that case. Do the military prerequisites drive the costs up more than the bulk production drives them down - comparing to civilian rockets?