I recently decrypted Telesto from an engram, and it only took about 10 motes of light to fully upgrade it. This is quite unlike every other exotic weapon and armor I have received. They have taken >10 motes of light to even unlock 1 additional perk.
(EDIT: Apparently I'm an idiot and had armor that benefitted fusion rifles, disregard.)
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Items (legendaries) should take 19 notes of light to fully upgrade I believe?