We took a Home James van last year (family of three). I don't recall what we paid, but they were reliable. They meet you at the airport terminal. Another option, I suppose, is Amtrak from Downtown Denver, though I haven't tried that.
I think Home James is your best bet.
Unfortunately, Amtrak is just not practical. Cab downtown, wait for train, cab in WP or just a 1 1/2 hour shuttle to WP.