what you get depends on what you can afford,nylon is a better fiber than polyester,but it cost more,what i would tell you is look at the warranty.(1) the wear warranty....means nothing,the way they are worded ,never going to happen.(2) stain warranty,stainmaster and weardated are the best,its a good warranty that the mfg's will stand behind (
3) no matt/no crush warranty, only the better carpets have it,it's the one i look for( you won't see it on many polyester carpets) if you can find all 3 warrantys ,you did o.k.
now as to price,poor quality $15/22 installed,good quality $23/28 installed anything over $ 30.00 is very good.
if you are shopping at home depot look at there "specials" ,basic colors ,but great buys....face weight,if its nylon 45 oz or better,polyester minimum of 60 oz....good luck