In my opinion it does nit compare at all. The knock off ones are usually much more hard and rougher on the skin. Also it soaks up most of the product. I would absolutely recommend to buy the original, it's definitely worth the money.
I tried a super cheap one from Forever 21 and it was HORRIBLE! Once you've used a real beauty blender there is no going back. I received mine free with points from an Ipsy bag and have since bought a second because they are so great. I've had my original one for like 4 years and it still looks new-as long as you clean it regularly.
Honestly, I've never found a knock off that I liked as much as I like the beauty blender. I know it's a lot of money for a sponge, but it is totally worth it. The beauty blender applies foundation flawlessly.