My experience with auctions for direct mail projects have been very positive. However, you'll need to do some selling of the process to both your internal clients and the participants in your dynamic bid.
Seven events that I have run for my current employer produced significant cost savings (10-37%) even for quarterly, repeated projects. Savings are produced by some obvious factors...tight, well communicated specs and pre-qualfied/"right-sized" participants, i.e., you can't have a small local direct mail provider competing in the same event as an RR Donnelley. The less obvious factors relate to how the concept is accepted by participants and getting your internal clients' support. When the concept of an auction is presented, the initial thought from both groups is "oh, you're just going to get a whole bunch of suppliers to bid on this just to get a low price". The buyer's pre-work to establish that the same decision points will be used in every step of the process as if the bid was going to be conducted in a more traditional manner is critical to the success and acceptance of the dynamic bid process. Your internal clients need to be educated that the only difference in the bid/award process is instead of receiving a single bid from several qualified suppliers (closed bid), you are going to create a more market driven environment that will force the same supplier base to more actively compete for the award. By the same token, your suppliers/participants need to accept that the bid(s) will not be open to any supplier that decides to participate and that the participants will be vetted and pre-qualified to participate. I can't emphasize enough how you, as the buyer, need to create the acceptance of an auction by all parties affected by the process and the results. It's all in how you position your effort. Best of luck.