I think the idea of finding out how much the best boarding would be in your area and then adding to that - would be a good place to start. You will be way better than boarding in a kennel - but if you set the price too high, you may not get the job.
My granddaughter keeps 2 cats (in her own home) for friends and they pay her $300.00 for 2 weeks. That's only about $21.40 per day, which seems low - but $300.00 for 2 weeks seems like a lot of money! Go figure.
Anyway, they are very little trouble, but the family on vacation is so relieved to have someone they trust keep the cats.