Harajuku Lovers items are really expensive, but that's for a good reason. Their items are considered as collectibles, since they only sell a few of each design. This is specially true for the bags. These are limited edition pieces.
However, I recently chanced upon a bazaar stall that sold HL bags. They were claiming the bags to be original and bought from Japan, but were selling them at only 1.5K a piece (big tote bags). I never really saw any fake ones before (in Greenhills tiangge or Divisoria and the likes) so I seized the opportunity and bought 3. When I got home, I checked out each bag against the previous bags I bought in Rustan's. Lo and behold. They were fakes.
First check out the zippers. It should have the HL logo zipper with a smooth metal finish at the back. Second, the HL print on tag inside the bag should be consistent with the HL font all over the bag. Third, it should also have the free bag charms like the keychains. Fourth, the paper tags indicating their style/item number all show the same item numbers despite having different prints, sizes and shapes.
I thought it was a bang for a buck when I bought the bags. It is a disappointing to find out in the end that the seller cheated on me. Nevertheless, the charm of the bags is worth it...

They don't really look fake from afar after all.

I suggest buying in Rustan's only. It's guaranteed to be authentic. Watch out for the sales (like I usually do, often during the -ber months). If you have a credit card, ask the salesperson if your credit card can avail of the 0% installment for 3 months scheme. Minimum purchase is 3k, which is just the average price of one adorable HL bag. ^_^