You're wearing a monocle?
If it's 16k...
9 charges -> 50 charges = 16,000 * 50/9 = 88,889
50 charges -> Unlimited = 88,889 * 2 = 177,778gp
Alternatively
177,778 / 2,000 = 88
spell level * caster level = 88
1 * 1 = 1
2 * 3 = 6
3 * 5 = 15
4 * 7 = 28
5 * 9 = 45
6 * 11 = 66
7 * 13 = 91
8 * 15 = 120
9 * 17 = 153
The effect is rougly equal to a 7th-level spell - you might be able to get a 1st-level spell or two like
true strike added to the effect for the same price.
Double RayLevel: Wiz/Sor 7
Casting time: 1 swift action
Target: Personal
The next ray spell you fire becomes two rays.
EDIT: If the item only works for eldritch blasts you can get a 30% cost reduction, to 124,444gp, or reduce the spell level to 6.