As you go up, of course the price goes up. I recently upgraded from SR80i's to SR225i's. The 225i's list for $200, though J&R Music World has them for $170 recently.
Comfort is the same until you hit the SR225i, which has bowl pads rather than flat comfy pads. Durability increases a bit due to a fatter heavier duty cable, but portability decreases for the same reason - the thicker cable is cumbersome. The 225i's come with a 1/4" plug rather than a 1/8" plug, so you may need an adapter. The screen on the 225i's are metal rather than plastic, so they are a bit more refined looking.
The difference in sound to my ears was mostly due to the different pads. The sound is different. All things kept the same, the 225i's have slightly more bass. The bowl pads deempasize the bass a bit but increase the soundstage.
The basic construction is the same through the 225i - metal headband with plastic cover, plastic cups. The 325i has cups made of metal which I've read makes them heavier which can impact comfort.
As an aside, I'm waiting for replacements for my 225i's because I noticed an imbalance between the right and left channels. I noticed this initially but chalked it up to getting used to the different style pads. I got used to it. Then, after not using the 'phones for a weekend due to being away, listening to them it jumped out at me again. The problem followed the headphones, so off they went to be replaced. I hope to have them in the next few days.
