HeHateY, this is a tough question. The only way that I know of to check out all (or most) of the fare classes available on your itinerary before purchase is on http://matrix.itasoftware.com/cvg/dispatch/ where all of the trips that it can find are listed in ascending price order. If you check these trips in the order provided, you can get its details and fare classes, but you will need to go over to another site in order to make the purchase. I have sometimes seen DL L and U fares only a few dollars more expensive than the Ts when I am using ITA to check out the best trips available for maximum MQMs.

Sometimes (more than occasionally) you will not be able to find the exact trip that you want on the airline site, and I have found that the best chance to find and actually buy it will be on Orbitz, which uses the same search technolgy as ITA Software. I for one am willing to forego the booking bonuses and pay the $6 fee in order to find the lowest fare with high MQMs, but many will not like this alternative. It may however, be somewhat of a solution to your specific problem