You normally get what you pay for. I ordered some cheap external battery packs - they were about $5 each - and they were terrible. I've since ordered an Anker $25 external battery pack and it has been great..
Got the 3850mAh battery today. The seller has raised the price since I bought mine. It seems he made a mistake with the price and he might have wanted it to be $199.95 now they are about $230. Glad I pulled the trigger when I did.