you have to consider the following:

• its not hard to be cheaper when you have considerable less license fees to pay (Linux, Apache, MySQL, PHP.. all free)
• often hosting companies offering huge amounts of bandwidth per month are connected to networks of lesser quality or lesser high speed connections to internet exchanges
• often they cannot deliver the actual bandwidth usage. Just imagine if they have 500 clients running per server, each 500GB per month, and 100 servers.. thats 2,5000,000 GB or 200,000,000 Gbit per month. If a month is 30 days, there are 30 * 24 * 60 * 60 = 2,592,000 seconds in a month. 200,000,000 / 2,592,000 = 9.65 Gbit. In other words, that company would need an internet connection of almost 10 Gbit. Sure.. you can get that kind of connection ... but would you be able to offer that kind of speed at such low rates?... I think not. It's the same as with buying a plane ticket really: if you are late checking in, you might get bumped to the next flight. The chance of that happening at low cost tickets is larger then with First class tickets....
• Have you considered the fact that PHPNuke is not free: a commercial PHPNuke license is €250 (from their license info page: http://phpnuke.org/modules.php?name=Commercial_License )

having said that.. if you search Google for "free dotnetnuke hosting" you'll get some links (although not as many as for free phpnuke hosting); http://www.google.com/search?hl=en&q=%22free+dotnetnuke+hosting%22

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