I think the idea that support costs will go down is incorrect, Apple is not going to provide support for your apps and if by getting an app in front of a larger audience more copies get sold more people will have questions (and they won't be the traditional 'power users' either they will need lots of help with even simple things and not take much effort to look at FAQs or tutorials for the app). I think have apps that do well on the Mac App Store will actually increase the amount of support requests.
As to the idea of dropping prices because users aren't used to buying apps for $40, I think this is also a bit off. Granted users that haven't bought any indie software aren't used to paying $40 for software, they are used to paying $80 for iWork, $50 for iLife, $150 for Office, $80 for Parallels, $100 for Photoshop Elements, etc. That doesn't even take into account pro level apps that are priced dramatically higher (just look at CS5). Take Photoshop Elements for example, when a user finds a graphics app on the store that will do what they need for $40 they are going to see it has a huge deal, especially if it is an established app that already has lots of good press on it's site. Games will be the only apps that really need to be in the $10-$30 range as that's where they are now, for the most part, from the established vendors that a user sees in boxes in the Apple Stores.
I can only speak from my own experience. Probably half of my support requests are related to installation, registration, lost serials, etc. I don't have those questions for my iOS products, and I don't expect them in the Mac App Store.
Re: price. Today, when you pay $40 for an app, you do it knowing you will probably be able to get future upgrades for $15-20. In the Mac App Store, you won't. Either you give away all upgrades free, or users will have to pay the full $40 every time. Somethings got to give.