Previously I said there are two ways:
1.follow the market
2.count every single affort you put on your stuff.
This is the exact rule. But what is the most important thing on selling stuff is to
Nobody wanting to sell items, where you put 100% of your energy / time on it, and later to suffer loss.
So, how to make profit:
1)If you doing your business seriously and wanted to chase for the wholesale market, BUY your raw material wholesale. You'll get raw at cheap price and now you can make profit out of your selling item. But remember, buying wholesale meaning you will get like a dozen meter of same fabrics or 1000pieces of same type of buttons. So, make sure the whole thing is no joke or you'll ended with no space to walk in your own house.
2)If you just making your business for spare time, buying your raw material in retail store is sufficient enough.
How to charge for an item:
The simple calculation is:
Raw items (fabrics + zipper + buttons + etc) + man power (how good is you) + time consume (how easy/hard to make the stuff) = You get the price
(Fabric: rm5 + zipper: rm2 + buttons: rm3 + thread: rm1) + rm5 + rm5 = rm20
(this is an average price for making a simple pencil case/ clutch)
OK, I hope this help. If you still uncertainty on how to put price for your items, listened to Rule#1: follow what other people are offering but make sure you're making the profits. It is good to be as same as other people, rather than being too cheap. People will start to query your product quality, and at the same time you're killing your own business (later how to survive if not making profit???).