Tethering is something that will cost more money than the 30/month data plan. It is another 20/month. However, I do not think you will need it.
I have owned the Droid X since it came out. Yes the 199 is a little much considering that I have not paid more than 100 for a phone in years. But it is a worthly investment.
I have replaced my Zune (mp3 player), my dell pocket pc (windows) and it also replaced a future purchase of a GPS unit. All that rolled up into one device. It would replace my Nook (ebook reader), but I just got the Nook this year and still like the 6" screen vs the 4.3" screen on the Droid.
My last phone was the env touch. The touchscreen on that was less than desireable, but the screen on the X is SO MUCH BETTER. rarely do I have a problem with my big fingers on this screen.
Also the X was the first phone I ever bought sight unseen. I am very happy with it. I have not had the any of the problems that you might read about on the forums.
If you do decide to get the X, I do recommend getting the car dock at the same time. This is a MUST PURCHASE if you are going to use the X for GPS in the car. The navigation function takes a lot of battery power because the large screen is on, the GPS receiver is on, and you are getting data from the 3g connection. The dock has a power connection so you have the car power to your phone the whole time. The only problem I have with the navigation is that sometimes it will go into the background. It doesnt happen all the time, or happen every day, but it can happen. It is easy to get it back (takes less than 3 seconds) but it is annoying if it happens.
I do have to charge it during the day at work, but I use it for mp3's, listening to internet radio, listening to podcasts and use it for browings the web sometimes. The large screen works wonders.
My opinion it is well worth the price, both the price of the phone, and the price of the monthly data plan. The screen is big enough to browse the web without tethering it to a laptop for an additional cost.
Enjoy