Do you have documentation that the arrows were cheap? I would be interested in seeing it, as the documentation I have found, some of which I linked to, shows just the opposite.
Again, in 1518 the documented market price for an english longbow was 3s. 4d., and price for a sheaf of arrows and furniture was 5s. 4d.
I can do basic math and understand basic logistics. Divide 5s. 4d. by 12. That's the price of an average arrow, including part of the furniture.
Then keep in mind that even in the modern world, militaries spend more on the average ammunition usage of the typical soldier than they do on the gun, and ammunition is still one of the most expensive aspects of fielding an army.
I wasn't certain how to word this so it didn't come across as at least partially sarcastic. So, I hope it was not too insulting.
Edit: To explain...
When you have something that is intended to fire repeatedly, you generally want the total ammunition usage to be more expensive than what is firing them. Massively more expensive. The ammunition can be relatively cheap, but military-grade ammunition tends to be a bit more costly anyway. The arrows used by a longbowman of the English army was probably a lot more expensive than that used by the average hunter in the woods just due to the basic construction requirements.
So, yes, you do want to be spending more on ammunition. Because otherwise, the ranged weapon itself is faulty and you add the expense of repeatedly replacing it on top of the ammunition costs. And you have to keep in mind militaries tend to have higher standards for their ammunition than the average person does because militaries are dealing with problems the average person doesn't see in their daily life.
With arrows, the expense difference is going to be more extreme than bullets. The typical arrow probably only has to pierce a deer or a bale of hay. The military-grade arrows have to pierce plate armor. The construction requirements between the two are going to be vastly different, with the military-grade having more rigid requirements and being more expensive due to the special construction requirements.
The typical arrow was probably constructed using whatever suitable wood was found in the forest, as well as whatever feathers were found. The only real expense was probably the arrowhead, assuming they didn't make that out of a rock or just sharpened the stick. As such, those arrows would be incredibly cheap... and also not traded, since most people could just make them themselves. The military-grade arrows, on the other hand, had specific wood requirements, specific feather requirements, had to use metal arrowheads, and so on. So those would definitely be traded.