I do additions to weapons in this way:
+2 damage = +1 CP
+1d6 damage = +2 CP
+4" range = +1 CP
-1 use = +1 CP
It's pretty balanced as far as I can tell. You can stack them as well.
I would do your missile in this way (and by missile I'm assuming you mean a rocket which does not need a launcher. If you mean an explosive you put in a launcher just use the adjustments above until you get the range you want):
Start with a 2d10 rocket which is 4CP, has a use of 4, and goes 12". A 6" rocket would go 36", so you need 24" tacked on like Bonn-o-Tron. 24 divided by 4 is 6, so you need to pay 6CP to get the extra 24". Your rocket now costs 10CP, deals 2d10exp, has a use of 4, but flies 36".