Who can recommend me an online jewelry store where I can buy an engagement ring with a beautiful diamond for a price that does not exceed 12 dollars?

Look at this engagement ring model, maybe it is just what you need http://pierrejewellery.com/katalog/oval-diamond-ring-3779y . It seems to me that your beloved will really like this ring. And the price should suit you. I am considering it for my girlfriend, most likely I will propose to her in January.