Most shadow boxes range from $15-$50 dollars, but because there are so many styles it's hard to put a good average price on them. You can pick them up at major retailers like Target or Michaels. If you have one of these stores near you, then you can just go there and pick it up to avoid any shipping charges. Being able to browse their selection online first wouldn't hurt.
Make sure you call them and find out if they have them in stock.