I went to local muffler shop (one that I found in yellow page ^^) and met with the sub-manager of the shop. I asked him the price for a custom cat back and he said that he can do the job for $60-65. I asked for the material of the pipe, and he said that it is aluminium steel coated pipe with rust protection and said that this material's quality is almost upto stainless steel's.

Anyway, when I install new cat back, should I remove resonator? When the sub-manager asked me this question, I just said to remove it...

 

The resonator keeps the exhaust quiet. I would keep it on unless you want a ricey sound.