Replica or knock-off clubs are cheaper because they are less in quality.

Yes, you pay more for name brands, but you increase in quality you receive is worth the price.

I've seen a large number of knock-offs, and have compared them to the originals.  If you are a once a month golfer, go a head an buy the cheap stuff.  If you golf on a weekly basis, or simply want to get better, then buy the better equipment.