It came out really well.
As far as cost you need to consider cost of each material. In your case 400cm for outer materal + 400cm of inner material, + thread, glue, dyes/paint, hardware, etc. You should also consider your overhead costs. What does it cost for rent, power, shipping costs (items you ship and items shipped to you), etc. And you need to consider man hours. What do you need to get paid hourly.
So you need; Materials cost + Overhead cost + Time
The biggest issue people have is figuring out what their time is worth. What you charge per hour should depend on on your level of ability. A person just starting out can't charge what a person doing this for 20 years could charge. So, If it took you one hour to make this then, with what you charged, your time is worth 9.50 an hour. If it took you 2hrs then you valued your time at 4.75/hr.
You also have to figure out what people would be willing to pay for the item. For that same item if you were to charge $40/hr USD and it took 2hr to make, most people wouldn't buy it. You would have to market it to select few that have the money to spend. It's a delicate balance you have to find.