If not the first receiver on the account, you would save $7 per month. The 722 is $17 and the 612 is $10. If the 722 is the only receiver on your account, you will save nothing as the first receiver is included in the programming package price.
The 612 is a dual tuner that serves one TV. You could go with a 211k. That's only $7 per month and for a one-time fee of $40, you can add an external HDD and make it a HD DVR.