My longarmer's rates for binding are as follows:
making and machine attaching binding (with fabric that you provide) for you to finish: $1.00 per linear foot (length + length + width + width, divided by 12)
making and machine attaching binding (with fabric that you provide) and finishing by hand: $3.00 per linear foot. (that's $50 for a 50x50 quilt)
Rates will depend on location (these are in NH).