The key is to provide a string bed that will feel better for the client than what they had before. Yea most people will want a basic $3 synthetic like Gosen or Prince, not a tournament nylon. Get a reel of synthetic, a reel of value multifilament (Alpha gut 2000), and maybe a few packs of a value poly. I would go with cyberflash. You want the best value strings, an the playability will speak for itself. You don't need to stock all of the good polys, since really good poly players can buy their own reel and cut off string for you to use.
Also, I would advertise prices as for labor and strings. That way your markup on one or the other is not so obvious. If someone provides the string, you can charge them like $12 since you can't markup the string.
(2) Dunlop Aerogel 200 (~345 g)- Genesis Typhoon (Gray) Full Bed (55/56.5)