Cost is a barrier, but time and interest are much bigger ones. Are there really that many DMs who won't be willing to pay $10 or so per month to get access to all the material on DDI? (Assuming it's high quality material) Those who aren't willing can always stick with Essentials. And I don't know about everyone else, but if my DM told me that he needed $10 per month to keep up with DDI, I would pay it myself. (Don't get any ideas, DM Dearest -- I know better.)
It's less a question of whether DMs will pay for it as whether players thinking about becoming DMs will pay for it. If there's an extra cost to becoming a DM, that's a disincentive to take on a job that too few people are willing to do as it is.
In the old days, a player interested in trying her hand at DMing could borrow someone else's DMG and Monster Manual and have a go. People are less willing to share their accounts and passwords. Moreover, buying a subscription upgrade is more of a commitment than the one-time cost of buying a DMG.
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