I am guessing you are searching for flights from Tucson (TUS) to Denver (DEN), but you can get much cheaper flights from Phoenix (PHX). I'm seeing round-trip fares PHX-DEN-PHX of $112 (Spirit Airlines, many extra fees likely), $144 (Southwest, includes bags) and $185 (US Airways, bags extra). The difference would probably cover a shuttle from Tucson to PHX (or you may be able to arrange a rideshare even cheaper).
Note that Southwest Airlines is a big player in this market (PHX and DEN are both big Southwest hubs and they run 9-10 nonstops per day), but they don't list their fares on the major travel sites (Orbitz, Kayak, etc). You have to go to Southwest's own site to see their fares. In this case it's worth doing.
Southwest will also sell you a nonstop TUS-DEN-TUS for $244 round trip.
To get from the Denver airport to Boulder, check out the public Skyride bus for which the fare is $13 each way.
Edit: If you are only traveling one-way, don't despair. On most US domestic flights these days, a round-trip fare is about the same as two one-way fares, so for each of the fares above, you should be able to get a one-way for about half the price. (It used to be that two one-way fares were far more expensive, or that round-trips required a Saturday overnight or some such, but those phenomena have mostly vanished.)