I just restring racquetball racquets, so not to sure about tennis racquets.
In my area it costs $35.00 to restring a racquetball racquet.
I offer my friends to restring their racquets for $10.00 including the string (Forten Competition Nylon 16g) or $10.00 with their own string. If they didn't like my restringing job, I would refund their $10.00 (cut out the Forten string) or their $10.00 (leave their string in).
Word got around that I did a good job of restringing their racquets for $10.00 and thus more and more players started to come to me for restringing. In the long run, people are smart enough that they would rather pay a lower price then a higher price for a good restringing job.
Still, there are players that can afford and want to pay $35.00 for restringing their racquets. My goal was to help out my friends and the average player who couldn't afford it and just wanted strings in their racquets to play with.
You know you are doing a good restringing job, when the club pro and other Open players ask you to restring their racquets.
In the long run, even without restringing for others, you will save money doing your own rackets and have fun doing it as well.