Does anyone know how much it would cost to have a plexi modded with a master volume? I wanna be able to achieve its incredible tone at lower volumes.
Well the pot is like $2 at the absolute most. Thats basically all that you need, but also the equiptment for it (soldering iron, solder, flux, wire, etc...). Then you need to drill a hole so you need a drill and bits. Oh yea, and you need skills to solder and discharge filter caps (unless you want to die )

get an attentuator
Parkwood PW320M
Epiphone Les Paul Standard
Epiphone C-40 willie nelson signature
Squier Strat
Some beat up acoustic from '68

Vox Ad50vt
Crate xt15r

Vox Clyde McCoy Wah
Saves you time and trouble, and sounds much better.

You won't get great Plexi tone with a master anyways, you need the power tubes pushed.
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.
I don't think you want to mod a plexi Marshall. (nor do you have one, no offense)

I think you're talking about modding a non-master volume JMP (or other Marshall) in which case this would also be a bad idea considering you can buy one. Look into Marshall 2203's and 2204's. They have a master volume. Spending the time and money to mod it wouldn't be worth it.