I'm not so sure about javelins being cheaper. The Pila, for example, is quite complicated (with that weak spear neck and all). A poorly made javelin is just as bad as a poorly made arrow, and there is much more material required for a javelin.

Just the wood itself would be an issue. An arrow requires considerably less mass of wood than a javelin, due to size. Finding the right wood of the right length and preparing it for a javelin would be considerably more effort than for a single arrow.

The only element I can think of that might make an arrow more expensive is the feathers. That cost would depend on what type of feathers that are used, whether the supporting faction has farms for birds for those feathers or not, etc.