Tax is going to be based off of the sale price, not the full retail price.
I can't speak for the other third-party sellers, but I bought my Droid Pro from Amazon Wireless. They seemed to have the LEAST restrictive "side agreement" of the third-parties. They're all going to make you agree to do something for 181 days. Some make you maintain the level of services (minutes, features, etc) that you currently have. Some (AmazonWireless, for instance) simply require that you maintain an active account on that phone for at least 181 days. That's not a problem, seeing as how you've already signed a two year contract anyways.
Here's the Amazon link ($50 with new account, $80 with upgrade)
AmazonWireless: Motorola DROID X Android Phone (Verizon Wireless)
Here's the key line for Newegg:
By accepting this Instant Savings Discount, you agree to repay $200 of this discount ($300 for smartphones and netbooks) and authorize us to charge your credit card in repayment of this amount if, during the 181 days after your new equipment is activated, your wireless carrier notifies us that the services you purchased have been suspended, disconnected, or deactivated, or if you return your purchase outside of the return and exchange provisions of our 30-day Satisfaction Guarantee.
In other words, you'll have to maintain your service plan at the level that it is at your time of purchase for six months.