Heh Heh, I Hear the word cheap and I think free or other peoples money. If your going to be using the room for both guitars and vocals but not at the sametime, leave yourself some variability. One thing you can do that pretty effective but rarely done is too build 2 foot by 4 foot slotted panels. One side of the panel should be reflective and the other with acoustic foam, so packing companies sell budget foam which shares alot of the same properties. Figure out how many panel you need to do all the walls not hidden by equipment, then make hangers where you need to hang the panels. This allows you to hang your treatments in various configurations to gain the amount of reverb you need for your specific application. You can angle them in the corners of the room to act as bass traps as well. Since your hanging them, they will move or float as low frequency "diaphagm baffes". This works very well and was used at Momentum Recording Studios in Seattle Washington, we could change to room acoustic for drum, guitars and vocals within a few minutes. One trick Ive done for amps and will be building another one soon is an isolation cabinet for the speaker cabinet. I put the mic in the cabinet on a small desktop stand that requires little space, I can move it if needed with little effort within 3 or 4 inches in all directions.
If your trying "soundproof" that would have have to done prior to the room treatments.
If youve got the time and want to spend the money, you can use exotic wood for the panels and Auralex type colored foam to beautify your creations.
Hope this helps :0)
Peace,
Dennis
