First time poster.
pg285 DMG3.5 shows a suggested formula for
estimating the price for a use activated or
continuous spell effect item.
2000 gp x caster x spell level x (4, 2, 1.5, .5)
For a ring of Invisibility
2000 x 3 x 2 x 2 = 24,000 gp
but the DMG lists
2000 x 3 x 2 x 5 / 3 = 20,000...