how much will a big ass window cost?
That depends on your isolation needs which translates into thickness. And that is determined by your wall leaf transmission loss(weakest link syndrome), and whether or not you have double walls or single. If single, it still depends on whether you decoupled one leaf, and if not, then if you plan on a single or double pane. One pane does very little for isolation if less than 3/8" thick. A 3/8" pane that size is BIG BUCKS!! Personally, Track Rat has the right idea for econo solutions. Unless you've already framed for your given size.
Don't forget seals, neoprene or EDPM setting blocks, and if you are using double panes, your window jambs must be structurally isolated from each other by a minimum of 1/4" (double wall), and DON"T FORGET SILICA DECASANT(moisture absorbant) between the panes. Also, it is advisable to thoroughly and correctly clean the glass on the inside faces(double pane) prior to installation and handle with gloves or vacume lifts. A 3/8" glass pane that size is extremely heavy!!
To give you a clue to the reality of commercial studio glass, most are a minimum of 3/4" and some even have 1". It takes that to equal the STC of high isolation studio walls. But again, this is all determined by your needs and db profile of the sounds you are trying to isolate. Trying to isolate drums on by a standard residential partition is rediculous. Even adding mass to each leaf adds very little in transmission loss(TL), so using glass that has a higher loss factor than the wall or door is a waste of money and time. Many people don't understand the "weak link" syndrome. Your isolation is only as good as the weakest link. Therefore, ALL the assemblies need to be rated in the same TL class.
BTW, Track is right. Unless you can find transparent aluminum
fitZ