Honestly don't think such a change would be needed. Siege-weapons are more expensive and slower in enemy land, that along with the actually good ones costing iron is definitely enough.
Later on the Archery-line also provides the supressive fire promotion, which gives them a spot to fill.
Supressive fire is ok to differentiate both units, but it comes a little late. When siege units have iron cost, there is a limit on how many siege units you can produce, so if you are short on iron or prefer to have ranged ships, then you are stuck with simple ranged. But with wooden siege units the higher cost is not reason enough. That early, unit maintenance is something to be aware of. Maintenance cost of a trebuchet is the same as the composite archer, so unless I need desperately more units (being at war), trebuchet is going to be my first option, even if I am not planning on doing any conquest.
See, the skirmisher is a ranged unit with a completely different role. It can be used to skirmish with hit and run tactics, it can be used to help with flanking bonus, chasing and pillaging (like any mounted unit), and other options.
A siege unit does all that ranged unit can do, and then something else. The slow movement in enemy lands is not such drawback for me, as most of my fights are near my borders. Being weaker in rough terrain either: first you need to position the ranged unit in a hill or in an isolated forest, losing movement points, only for that unit to be a little more resilient (neither would stand a knight charge). On a forested area without hills, ranged cannot match the indirect fire promotion of siege. So, most things that a ranged unit can do better than a siege unit (move faster in enemy land, defend better) are not enough to use it in a different role. Supressive fire is the only thing that encourages to use the ranged unit to cover the back of melee units and this comes ... with machine gun? Ranged units have access to healing promotions, but so do scouts and any melee.
What I am saying is that, in order to make production of ranged units more desirable, its supportive role should be encouraged earlier, at least with composite bowmen (early archers can be used for rushed attacks, in a time where no siege units are available, so that is sufficient), and strongly, with one or two more promotions dedicated to the support role.
I dislike the idea that a ranged unit is a cheaper siege unit (doing the same comparison with spearman-swordsman). Without siege units is almost impossible to take inland capitals, so ranged units cannot be considered their cheap replacement. I am going to produce siege units either way. But I can skip ranged units, as siege units can perform quite well in defense too, just staying out of enemy range.
