Cost to treat a room?

One way could be to buy and read a couple of books (ej, Everest's and Floyd's books) to understand what's going on, get an SPL meter from Radio Shack ($60?) and a CD with test tones and start experimenting with home furnishings, comforters, etc to further understand what might work in your room. Then you could either buy or DIY treatments (receipes available at AAsylum, for instance).

I'm going down this path and while it's been a lot of work it is being fun and I'm learning a lot as I go. It's likely the cheapest albeit most time-consuming approach.

My two cents. enjoy!

http://www.readyacoustics.com/

$815.95 maybe?

Also another question I have is, if I treat the room, then change my amp/pre, would I have to make any adjustments with the room treatment? All audiophile amps/pres are supposed to give a flat response, correct? So my guess is I would not need to change anything with the room treatment or remeasure unless I change the speakers. Is this true?

Yes it is true. BTW Room treatments do not really change frequency response of audio equipment so much as room treatments will reduce or remove the audible effects of reflections off the walls. That can make it sound better or not depending what the reflections add to the sound that you find objectionable or pleasurable..

If you treat your room properly you should be able to tune around system changes by adjusting speaker placement and toe in. If that doesn't do it then removing or adding treatment can be done.

After treating your room you are most likely going to want to play with speaker placement anyway as most likely you are compensating for the sound of the room with your current placement and toe in.

You can fake room treatment with a few pillows just to see if you want to put the effort in. I myself wasted a bunch of money in years past on 'warm' cables when what I really needed was treatment behind my speakers and at the mirror points and a touch less toe in. Ended up getting rid of the brightness and huge improvements in detail, soundstaging, and pace.

I would read up a bit before you do anything.
The Room treatment vendors sites that have been recommended have a lot of info. I also like Jim Smith's book "Get Better Sound".