New tires cost $500 to $900 a set, so they belong in the price you compare (sample guide).
Put a quarter upside down in the main groove. If you can see the top of the head, the tread is under 4/32 and the tires need replacing soon. Check the inside edge as well as the outside: uneven wear points to the alignment.
Look for the four-digit date code on the sidewall. Tires older than about six years harden even with tread left. On our sheets the tread depth for each wheel is written next to it.