First, you will have the additional monthly cost regardless of where and how you get the new receiver. First receiver (your DVR) is free, subsequent ones are $5. Does not matter if it is leased or owned.
Second, the standard cost for a leased HD receiver is $99. Sometimes you can get them from DirecTV for less. Receivers from eBay are very "iffy", unless you are getting them from a recognized DirecTV retailer. Those boxes are leased, sometimes you can get them for slightly less than $99 (including the access card). DirecTV boxes from non-approved sources on eBay are often dubious. Most HD boxes are leased not owned, and if you get one of those DirecTV can (and usually does) refuse to activate them because they should have been returned to DirecTV. To be safe, you should always get the receiver ID # (and the access card number if you can) and call DirecTV, ask for the Access Card department. They will tell you if the box is leased or owned (don't take the word of a regular CSR) and also if the account for the box is paid up. If the box is owned, you can buy it. If it's a used receiver, you will need a new access card ($20) so it's often not worth it, you might as well get a new one.
Also, keep well away from H20-600 receivers. They overheat and many of them have already been damaged by prolonged overheating.
With all of that, IMHO it's not worth doing.