True, but that's no reason for an item improving rays to be overpowered. In fact, an item that only improves rays should cost LESS than an item that improves all attacks- like Bless or Heroism- because Bless and Heroism improve ALL attacks, INCLUDING rays. If I remember correctly, there's an Ioun stone than gives you a +1 competence bonus on all attack rolls, skills checks, and ability checks- an item that only improves the attack rolls of rays would cost much, much less.
The only existing item I can think of at the moment is the Sorcerer's Hand crossbow from the Arms & Equipment guide. It's a +2 light crossbow shaped like a hand with two outstretched fingers- and, if you use it as an additional focus while casting a Ray spell, you gain a +1 or +2 bonus on your ranged touch attack roll (I can't remember which it was- I don't have the book handy) and your caster level is counted as 1 higher for purposes of that spell only. If you have that book, you can use the cost of that weapon as a guide- just take away the +2 enhancement bonus from the weapon, and see what you're left with for the price.