Only changing the muffler will have a slight change on the exhaust note, but I don't think its worth the money just doing that. Usually when they put a 2.5' exhaust on they put a bit smaller 'sports' muffler than the stock one (which is in the middle of the system) and have no resinator which is the big looking tin can at the back of the exhaust. This will give you a lot deeper and louder note.
You could take out the resinator and put a straight piece of pipe in, with also putting a sports muffler in. This will give you a fairly deeper note. Its basically what they do when they put a 2.5' exhaust on, but you will just have a 2.25' stock pipe.
If your looking for the cheapest option for a deeper note, just ask an exhaust shop to remove the resinator and put a piece of pipe in which will cost like $50.