I have a voice amp sorted now, but when I was in your position (thinking about how to do it all on a shoestring

) I thought about what I could adapt. I figured that actually physically making an amp set up would be a) beyond my meagre skills, and b) as, if not more, expensive as buying a radioshack/yoga amp.
My best idea (never carried out) came from my daughter. She has a fisher price cassette player with a mic, so she can sing along to her tapes. She started talking through it one day and said it sounded like Boba (she's 4 and has watched ESB more times than I care to work out!) I believe her exact words were, "listen daddy, I sound like your Bobo Fat" (her name for Boba.
I tried it and it's not hugely loud, but it's ok and it's a start.
Try scanning ebay for cheap kids toys that have mics and amps.
Just a thought.
Hope this helps.
Si