IMHO, you get what you pay for. I think you answered your own question about the cheaper carpet. There are a lot of factors that go into carpet selection and wear. Nylon is best but there are other factors I'm not going to get into. Search the web.
I see carpets going into houses from $7 to $17 a yard. Pad runs $2 to $4. Installation runs $3 to $5. People who buy cheaper carpet usually buy a cheaper pad. Although carpet life and wear also depends upon your lifestyle, traffic volume, etc.
For instance, if a $14 carpet lasts twice as long as a $7 carpet compare the costs. $14 (carpet) + $7(pad & install)=$21 a yard. $7 (carpet) + $7 (pad & install)=$14 times 2 (for half the life)= $28 a yard. Good luck to anyone trying to buy carpet.