business - How to accurately / competitively price prints when digital images are already paid for?

My wife is a wedding and fashion photographer (as well as some family and maternity). We've been traditionally giving our High Res images as a part of the package price to our customers. Now that we've started using SmugMug for our client gallery, we're thinking about offering prints as well.

The problem here is that because we've been giving the digital copies for free, and because a client could take those digital copies to the cheap spot to get them printed... we're unsure where to "start" when it comes to print costs.

One thing we are considering doing is taking one of our images and getting it printed at SmugMug in "Gloss" "Luster" and "Metallic" and then also get it printed at Wal*Mart, Costco, London Drugs, Blacks, etc. This way we'll be able to show the client the difference in quality.

My question is, how can we go about pricing our images accurately and competitively? We want to be competitive, but we don't want to deter our clients from using a professional print lab over Wal*Mart.

Edit

Though all of the answers and comments thus far are very helpful, I feel the focus is on the fact that the digitals are free. I'm wondering a little more about how to price the prints.

Answer

The market is changing. Many people can make their own prints - as it is now nearly trivial to do so with inexpensive-but-excellent 7+ ink home printers - and will look for this option when hiring a photographer. More people know how to operate computers. Photographers, usually the older ones, hate this.

The photographer is tasked with taking the photos. You may wish to choose another person to create the prints. Of course, this goes against the current wedding photography motto (which goes something like "extract as much from your client as you can by instilling in them a fear that this most important day will be ruined if they don't spend enough").

You can find yourself enough clients that will take highres jpegs and do with them as they please. The idea of managing licensing options for wedding photos is too complicated. Commercial photography is one thing, wedding another. A couple wants to make another print for their parents - they should not have to contact the photographer.

The most sincere and transparent way to approach pricing is to ask yourself - would I pay this much for a print? If you say 'no', then reconsider your market and your approach to your clients.