105 > ultegra> dura ace.
The bike I bought just prior to moving to Manhattan was a Trek 1100 with rx 100 group set(had the down tube shifters). I just googled it and read it was comparable to a 105 at the time. I did long tours back then, and was already doing 60 mile rides. My earlier bike was an even lower end Trek with a lower end group set(a 330). I always felt that upgrading to the 105 level equipment was like crossing the rubicon. Most riders if they are serious will at the very least wind up with 105 at the least but most likely ultegra. Dura Ace is mostly over-kill and not necessary by any stretch.
You need to reverse those mathematical signs, 105 is less than, not greater than, ultegra. (I know that you know. Don't ya just love math
).
OP, dura ace is the highest shimano component. 105 is considered entry-level race by some. Personally, I buy at least at the 105 level. If you decide you want to pay X number of dollars for a particular frame set and group, and then set about finding it, you should be able to stay within budget, and get the group you want.
105 is very good, and you will probably not feel compelled to upgrade. Whatever you do, avoid paying full retail. In fact, I never have, and the lowest component group I've owned as an adult has been 105, and I've had zero problems with it.
