It looks like the Essentials Dungeon Master Kit for 4e is hard to find new, and the new copies I can find are really expensive. Is this because it is out of date or out of print? Is there a better/updated item I should be looking for instead? The monster vault, red box, player compendium, etc are all easy to find and affordable so it is strange to me that this isn't.
This product, the Dungeon Master's Kit from the Essentials line, was first released October 2010. Since the announcement of DnD Next, Wizards has been slowly phasing out 4e products. While I can't find any official reference to the product being out of print, I suspect it might well have been quietly removed from print.
It might be easier to find the Dungeon Master's Guide, which is not in the Essentials line but instead comes from the main line of products and should contain the same rules. The Deluxe DM Screen is in stock at Amazon, and the dungeon tiles also seem relatively easy to find if you're looking for map resources. You've already mentioned the monster vaults; some other adventures, like "Madness at Gardmore Abbey", also come with battle mats and tokens that might be useful.
You can now get it as a PDF online
The Dungeon Master's Kit has been added on DriveThruRPG.com
If you're a Dungeons & Dragons player looking forward to your first experience as the Dungeon Master, this box contains everything you need to get started, including expert advice on making your game the best it can be. Create monster-infested dungeons, gather your friends, and let the fun begin. In the Dungeons & Dragons world you create, anything is possible—the only limit is your imagination!
- Dungeon Master's Book, containing everything you need to build and run exciting D&D adventures
- 2 books containing "Reavers of Harkenwold," a ready-to-play two-part adventure
NOTE: This is a digital download product and does not include the double-sided, fold-out battle maps; the 3 sheets of cut-out hero and monster tokens; or the fold-out, four-panel DM's screen that were included in the original physical product.