At marsmusic.com there are a few marshal mics that are under $100. I would think that most of those would be better for vocals than the sm57. I don't understand why people who own a 57 would recommend them for vocals. For electric guitar amps, they're nearly unbeatable (try a c1000 though, if you haven't already, especially for clean tones), for drums they're fine, but for vocals... almost any condensor mic is preferable imho.
Both the V57 and the V63 are available from Mars for $79. If you're just doing vocals I would recommend you look into one of those, or maybe wait for the Studio Projects B series to come out. I haven't used any of those mics personally, but I don't think that you could go too wrong for the price. In any case, they're going to capture vocals more accurately than a 57. The high end on a 57 is really inadequate for most vox, in my opinion, although some people have claimed to get good results with them. I guess I just really, really don't like the 57 for vocals. It's hard for me to imagine that those Marshalls or the SP b series could be any worse.