cheap mics for close-micing guitar amps?

Just get a real 57, seriously it's the best bargain going already. There is literally no reason to get a copy instead
 
If you're getting a used 57 (a la ebay) watch out for fakes.

used ones are usually ok. As most of us know, you could literally throw a 57 out a window and it will still work so buying used 57s is actually quite shrewd, they're cheaper and the liklihood is they're still going to work as well as they did the moment they were bought. The 57s listed as new on ebay are the ones to avoid!
 
Best to buy new from a genuine sure dealer .
I cant recommend buying from eBay . There are ways of telling genuine from cheap repo,s but their subtle at best.