Standard charge for an HD receiver form DirecTV is $99, there is no "swap" as such, you don't get any credit for your D12. If you login to your DirecTv account, you might find you have an offer of an upgrade at less then $99, even free. With DirecTV, you can't specify what sort of receiver you want. If you want a specific type, you would have to get it from an online vendor like Solid Signal. Typically same price, still leased, still a 2 year commitment.
On ebay you will see receivers from third party authorized distributors, typically for the same price as DirecTV or sometimes slightly less, still leased and 2 year commitment. You will also see receivers for "sale" from individuals. Buyer beware, many of these are leased receivers already on someone's account and DirecTv will not activate them! They will only activate "owned" receivers, and a typical price for an owned receiver on ebay is $150 or so. If you want to buy from ebay, ask the seller for the receiver ID and then call DirecTV, ask for the access card department, and they will confirm for you if the receiver is owned and can be activated. Otherwise you are buying an expensive doorstop.