go to walmart and buy some eggshell foam. to put it up over an entire room should cost you less than $50. probably more like $25.
you don't want to use bounceback as amplification, which seems to be what you're getting at. couple reasons here... of paramount is that bounceback soundwaves will not have the clarity of a direct wave. if you want reverb, use a reverb filter. you can always undo effects, but if you record with one, it's there forever. likewise, if you're using it as an amplification device i can just envision your waves looking like clipped out blocks instead of a nice variety in amplitude. put the mic in the center of the room, or if you're in a corner, spit away from the corner and not into it. rule of thumb is that the farther your mic from your face, the more roomy it will sound, and this isn't good for lead or sole vocals. get a spitguard. buy the eggshell. assuming you don't have a crappy mic, you're good to go.
we had to do an improvised booth once to meet a deadline, and it cost us about $25 to do it. we used a hotel closet, bought a pack of nails (used a heel and cheaped out on the hammer) for $5, bought a flashlight for $3, some nylons for $2 (spitguard), and bought an eggshell mattress cover for $15. went to best buy and bought a $200 mic, which we returned the next morning. done deal.