Cost to treat a room?

GIK is great for the money. Real traps start getting pricey but they are well built, their web site is worth a visit just for the info on room treatment they provide.

Just start with the mirror points on each side wall between you and the speakers. Second is treatment behind the speakers.

One thing to think about is that you can't EQ bass room nulls, and using an eq to reduce peaks does not sound the same as reducing them with room treatments...a treated room will have faster time response.

So you may want to consider doing a low frequency treatment in corners behind the speakers in addition to the EQ.

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".