personally, I think $50 for a dress (presumably handmade?) with lots of detailing is already priced way too low. The customer is not asking for a duplicate dress you have in stock, they are asking for a custom order. Custom orders are not priced the same as regular, off-the-rack items. If she wants a custom-sized dress she needs to pay significantly more. There will be a new pattern to create in her size, fittings, shopping, more labor.

IMO, you do not "need to accomodate her" nor "at the same time make her a $50 dress". :)

I'd tell the customer that you are thrilled she's so interested in your work, but unfortunately that fabric is no longer available and you are sorry you are unable to accommodate her at this time. Tell her that you hope she returns to your store again and that for future reference there is an additional fee of $X for all custom orders, such as custom sizes. If you feel she's been some how inconvenienced you could offer her a coupon for your store for her next purchase. In general, I don't feel you owe her anything. Simply be polite and professional about not fulfilling her request, stressing that you value her as a customer. You could also stress that you feel part of the beauty of the item is it is one of a kind, making it special for the person who does own it. So while she may be frustrated now, she could potentially own her own OOAK dress from you in the future. Of course, phrasing it in appropriate customer-speak. :)

I am in agreement with Ms. Dynomite... and also ask yourself: do you have a return policy? if not, let her know there are no returns, and hopefully she will find someone who fits and likes the dress. if so, have her return they dress, let her know you would consider making a custom dress, but she will have to come in for a consultation about the order. That's when you can go over the fees for measuring, fitting, sourcing fabrics, presenting a selection, making a muslin, first fitting, second fitting, final fitting. She might not be so demanding on fit and color once she realizes how much time and money is involved for her. And if she does have the time, money, and interest, you have a new client!

Regarding fees... you have to pay yourself first: pay for the design, pay for the resourcefulness of finding fabric, notions, etc. pay yourself for your skills and your skill level, even if the garment doesn't require high skills... pay yourself for the time you take to design, sketch, draft pattern, clean and cut and press raw material, the time to assemble and adjust and fit the garment, the time for finishing the garment with snaps and hems and labels... the time it takes to get to market (website, craft fair, barter), the time it takes to do the paperwork/bookkeeping, the time it takes to package the dress, label the dress (hang tags?), bag and ship the dress.... then double it for wholesale price... and 4x it for retail price.

that is the basic pricing structure for off-the-shelf products