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.)
-
Armor takes a lot but pop a telemetry and exotics and legendary weapons level up like crazy. My sleeper simulants took 5 motes each.