I'm A Banana
Potassium-Rich
The players in my Tiranny of Dragons campaign acquired the intelligent sword Hazirawn and decided to use it. The sword is evil and it is currently held by a paladin, so it has been unable to charm him, but it started to complain and speak out in very bad moments. For example, it got the party into a fight the dragon at the end of HotDQ, while the players were trying dupe him. I thought it was very clever on my part, but one of the players almost rage-quit. Go figure. So the party wants to have a magical scabbard crafted, that will silence the sword as long as it is in it. I looked up the crafting rules in the DMG and I figured it would cost 2000 gp all included, considering the scabbard should have to be made with expensive materials to start with. So the artisan could start by asking 4000 gp, and drop down the price a little if the negotiations go well.
Any advice or suggestion on all the above?
You can do that if you want. Seems a bit dull to me to just pay to get rid of it, though, personally.
I would make it a quest.
I would make it a quest that has to do with the history of the sword, the paladin's order, and that ties back into the Tyranny of Dragons storyline.
So, Hazirwan is Rezmir's sword, and it speaks Netherese. Not too much of a stretch to say it was made in Netheril. Netheril is one of the big antagonist groups in FR, and their ruins are all over the frickin' place. Maybe Rezmir found it in the early days of her assembling the Hoard. The easiest link would be to the Red Wizards - perhaps Azbara Jos and the sect of Red Wizards working with the Cult wanted to learn more about where Rezmir found Hazirwan, so they could pump it for magical secrets from the fallen empire.
Of course, now, the PC's have the chance.
As the party discusses getting a scabbard of put a sock in it, you glorified can-opener, they hear of a set of eerily silent ruins only a few miles from town (maybe Parnast, if you're that far, but really, wherever, it's not like Netheril wasn't all over the Sword Coast), where adventurers and (recently) Dragon Cultists have disappeared. Maybe the 4,000 gp they pay gives them that link - a map that leads to this location. Rumor has it that there's some powerful magic in there, generating a supernatural silence effect. As is often the case in Fantasy, rumor is basically right - these are Netherese ruins, and the silence effect is part of a prison for a phaerim, an effort to starve it of magic (the Paerim, though weak, is still alive and malevolent). The party can, indeed, bind the silence magic to a scabbard of their choice (a brief ritual outlined in the faded documents in the prison shows how a spellcaster can move the effect to different objects - but such things are not guaranteed, and take Arcana checks to pull off), but doing so will reinvigorate the phaerim held captive there.
If the party can find the ritual, conduct it, and slay the phaerim that escapes before it ravages the nearby countryside, they'll have themselves a nice little scabbard and maybe some bargaining chips with the Red Wizards (who are VERY interested in this story).
If they can't find the records, or they mess up the ritual, Hazirwan won't ever really shut up, so they'll either have to put up with it, or get rid of it. If the phaerim escapes, the party might have a bigger problem on their hands after the Dragon Queen is dealt with...