There is no one definitive answer to this question as the cost of a rare back bling can vary depending on a number of factors, such as the specific back bling itself, the current market conditions, and the seller. However, as a general rule of thumb, rare back blings tend to cost more than more common back blings.
If you are interested in purchasing a rare back bling, the best way to get an accurate estimate of its cost is to search for it on a site like eBay or another online marketplace. You can then use the "Sort by: Price + Shipping" feature to see a list of all the available listings for that back bling, from lowest to highest price. This will give you a good idea of the current market value for that particular back bling.