I've always assumed helmets were part of the armor set. It says so for plate, although, it doesn't really mention it for others.
a suit of plate includes... a visored helmet (PHB, p.145)
Having a separate helmet for an additional 1 AC for 10 gp is mechanically equivalent to giving the players +1 armor for an extra 10 gp. In the DMG, +1 armor is listed as a rare magic item. Getting the same effect for 10 gp is pretty unbalanced; since all of the to-hit modifiers are assuming that characters don't have this.
As the DM, it is up to you how you want to flavor your world. If you wanted helmets to be separate, I would suggest either not having them add to AC, but, rather, act as sort of a prerequisite for interesting attachments (like the mask you were mentioning), or treat them as mentioned above for light armor, but reduce the AC and cost of all heavy armor by 1 and 10gp respectively.
Have medium and heavy helmets add 1 to AC, but impose the Dex cap. This would make the helmet optional, not change the balance much (other than for the attachments) and give you your versatility.
I would personally further tweak it to be higher than 10 gp so that the option of helmetless plate was more attractive, but that's up to you.