Sorry for the really slow response JS.
If you root your ET, and get a tether app, no charge. A little bit of work (but not much) on your part.
If you go Sprint's "route", Menu, Settings, Wireless and network, Tethering... you'll pay Sprint. Monthly.
Disclaimer: There are arguments on whether or not this is an acceptable practice under the terms of your contract with your carrier, the details of which are better left for another thread. (There are plenty you can read if you like). I choose to tether on my own. I also am cautious about the (amount of) data I use. You can make your own choice.
Google "android tether". Click the code.google.com link (it should be first). You should be able to follow from here. But your phone must be rooted first.
Alternatively, search 'tether' in the Market - lots of apps that will do the legwork for you.
I got the NC because I like to read, and books get heavy

, but I also knew I'd be rooting it pretty quickly. (I think I waited a couple days.) Very glad I rooted it... By itself, the NC is wonderful. Rooted as a full-on tablet, it's 20x better. Kindle App, Overdrive Media Console...
Good luck!