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Are you certain the spot is on the inside and not some small amount leftover from the spot you cleaned? Maybe try a different brand of lens cleaner, but make sure it is safe for lenses so that the lens coatings are not removed. Even weather resistant lenses can get dust inside, the seals can only keep out so much dust and zoom lenses are known to get dusty on occasion. More so than prime lenses do. Weather resistant means more sealing, but achieving perfect sealing is tough to get. It is also possible the dust has been there from the time it was manufactured and you are only now noticing because of the white dot. Does the dust show up in any of your photos? As far as cleaning cost goes, your local camera repair shop will be able to give you some estimates, prices will be different depending on where you are.

You may have to wait for next Easter and try again then.