My take on it is this: your customers should be paying you for the added value of having something meaningful on that sheet of paper, for your expertise in capturing a moment. You shouldn't rely on customers' ignorance for them to purchase your products, but rather, you should try to make images that they feel are worth the cost you charge.
And as for that, I think a customer who is curious enough to go to www.zenfolio.com, click on the Mpix link, and then read all the way down to where the prices are (it's a _long_ page, and the prices are by no means at the top) might also be curious enough to Google for "mpix" and click on the convenient "Print Pricing & Sizes" link, on which page the prices _are_ at the top.
If you're still concerned, though, then host your site at a custom URL and click the checkbox to hide vendor labelling.
--xsdg