Soundproofing apartment recommendations - help needed!

[Mary S.]

I own two units next to each other (live in one) and want to sound proof the two common walls between the two since you can hear from one unit to the other.

I am very confused about the best soundproofing material. We need to install additional sheet rock to the existing wall with two inches in between. I am confused what goes inside. Is Roxul Safe and sound sufficient? What about using sound panels, or a lead sheet to line the wall under the sheet rock?

How much is needed without being exorbitantly expensive?

Thanks in advance!