How much cost to replace carpet?

Measure the area of the space to be covered. Multiply by the cost of the unit area of the carpet. A typical economy carpet at Lowe's is about $2.25 per square foot including pad, plus $97 for installation of any number of rooms. A pretty nice carpet runs about double - around $4.50 per square foot.

If you need only carpet for one room (up to about 12' by 18') a remnant store is your best bet. A typical one room price is about $150 or so plus installation.