Hi, have you met me? I am OCD about all this crap. You can scroll to the end to get a really simple formula for charging, or read the OMG you're nuts version.
But:
https://www.youtube.com/watch?v=q8mkValuei0Shows how to make a cost sheet for RTW, but principles can be applied to one offs.
A major problem I find is that the cost of material+labor+overhead=too high, especially using retail sources. And forgive the American prices, but all of these are resources anyone can use, they aren't my wholesale accounts.
So we are going to assume that for the same of argument, that you got the fabric from Fabric Wholesale Direct (so I can get a price on it) and $4.99 a yard is the price point. You need 2 yards. Now going to Wawak for the rest: bias tape is $1.40, need 2. Add in a little for a needle and thread (needles cost me about .10 each for a domestic) for a total of about .15. You said 3 hours labor, and you've made it before, so we can kick out the pattern cost, which leaves us with sewing, laundry and cutting. At $10 an hour, that's $30, plus laundering $1. I am leaving off the shipping here, because we probably won't need anything extra out of pocket to pay for, other then the actual cost of shipping.
Fabric=$10
Trim= $1.55
Labor=$31
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Total=42.55, round down so $42
At this point, you should multiply that by 1.8 or 2.2 for a "wholesale" cost, but it is already pretty high, and you're not in it to make money so much, or you can round it up to $50.
There are a few issues sewing professionally: you tend to take longer learning how to do things faster, so you tend to take a labor cut. So using the same formula and fabrics for my business I would get $26-30 as a cost, including my 1.8-2.2 mark up (about $4 for overhead). Main difference is that
a) I have a cutting table most don't have, so cutting is a matter of setting it down and I use a cutting knife for simple things, which makes a cut out less then 10 minutes.
b) I would make my own bias tape. I have tools to make the cutting of it down to mere minutes (it's a ruler, rotary all in one that does all my work once I find the grain) and I do this often enough I can make 10-12 meters/yards of it in about 15 minutes from cutting out to finished. (Yes, I make a lot of bias tape.) It does not add considerably to my labor.
c) I have binders that make the bias tape attachment go fast.
d) I can sew this in about half an hour, or should at industry standards, making my labor affordable. Same as the video spreading the labor costs in a way.
e) I use industrial machines.
So using tools, experience and repetition, I cut my labor down to 1 hour for it all, and honestly, it may take me more time, depending on what I am doing that day and what I got going on. This is the best case scenario.
Mostly, you don't want to be out of pocket for anything. When you aren't as OCD and I am, it can be simpler and you can charge 2 or 3 times the cost of materials to make it (in my scenario, it would be about $24 to $36).