Could my guitar amp be used as a temporary makeshift mic speaker for my band?

Ive recently started a trio band and I got a mic and mic stand practice singing while i play guitar by myself. I have a 30W spider III and a 120W spider II. the mic i got isnt an expensive one($10) because i didnt know if id use it alot. I plug the mic into my 30W and play guitar through my 120W but before i did this i asked a question on yahoo about this and the person said watch out cause the mic might be loud through the amp. But I plugged the mic into the 120W and it was much louder if louder at all than the 30W. I need to be able to sing in my band and we have another 100W guitar amp but is there anything i can do to make the mic louder? buy a better mic? because we dont want to buy a really expensice PA right now. we just started. Any help on ways we could use the amps as a makeshift PA?