Prices for those models should not increase. I bought my child safety seat a while ago. Once the tether anchor hardware is installed in the vehicle, tethers are very easy to use. In most cases you simply connect a snap hook to the vehicle attachment point and pull the strap tight. Read the vehicle owner's manual and child safety seat instructions for specific directions.
Most vehicles built in the past ten years have tether anchor points built in.
Read your vehicle owner's manual to determine these locations. The attachment hardware can be obtained from your vehicle dealer. Forward-facing child safety seats without tethers are effective if used properly. Be sure that the child safety seat is installed securely so that the bottom of the seat doesn't move more than one inch in any direction. Also be certain that your child is properly strapped into the child safety seat.