At what cost Melkor?

[Ancalagon]

And there right before her was the great head of Glaurung, who had even then crept up from the other side; and before she was aware her eyes looked in his eyes, and they were terrible, being filled with the fell spirit of Morgoth, his master. The Children of Hurin.

And he cried in mockery of Glaurung's words at Nargothrond: "Hail, Worm of Morgoth! Well met again! Die now and the darkness have thee! Thus is Túrin son of Húrin avenged." Then he wrenched out the sword, and even as he did so a spout of black blood followed it, and fell upon his hand and his flesh was burned by the venom, so that he cried aloud at the pain. Thereat Glaurung stirred and opened his baleful and looked upon Turambar with such malice that it seemed to him that he was smitten by an arrow; and for that and for the anguish of his hand be fell in a swoon, and lay as one dead beside the Dragon, and his sword was beneath him. Unfinished Tales

At what cost to Melkor was the slaying of Glaurung? Bearing in mind his spirit is partially imparted into the Dragon to allow him existence, does Morgoth feel in himself a weakening at the loss of Glaurung and therefore, ultimately a loss in himself?

[YayGollum]

Well, filled with the spirit of Morgoth could merely mean the same thing as filled with a spirit of malice, Morgoth being a particular spirit made out of malice. Filled with the influence of a spirit of malice. Is much information given about the nature of extracting spirit energy and injecting it into whatever raw materials were used to make Glaurung? Anyways, I am of the opinion that, yes, Mel felt his power diminish when Glaurung died. I figure that he felt his power diminish whenever any of his creations died, which would include, at least, the original Orcs and dragons. I am not certain that Mel lost any creepy spiritual energy, though. Is raising a giant lizard who already came with magical powers to be evil much of a way to lose spiritual energy? Sure, mayhaps he put a lot of energy into raising the guy. Mayhaps Glaurung's upbringing could be considered a great work of art by Mel. I can't be sure, though.

[Grond]

The Silmarillion is ripe with references of Melkor "milking" or "leeching" his powers. Remember that he hadn't the ability to "create", all he could do was "distort" and "pervert".

I don't think he lost anything at Glaurung's death. I'm pretty sure that the power he lost was when he "perverted" whatever creature was Glaurung's ancestor. (probably a toad of some kind ;)).