Sewing is a skill, even if you are sewing a straight line but you are also having to keep two pieces of material together at the same time. You'd be somewhat surprised at how many people can't do or won't sewing. Fine sewing is learned just like being the plumber, or guy/gal who repairs cars and look how much they charge for their skills. I have done some custom clothes (bridemaids) and I tell the client up front how much I will charge and it's up to them to decide and I require a down payment. Get a clean understanding from your client.