It's most likely a fake. Signed prints by Haring and Warhol for for a lot more — you can get an idea for how much by looking at Sotheby's/Christie's past results.
I'd be surprised to learn about an unlimited run of a print that is signed by an artist like Warhol or Haring. The lithographs in your sibling comment are in the price range you're talking about, but have a printed signature.
