You can start developing a price by finding out how much time each piece takes you, then multiplying that by a decent wage. Start with, say, $20 an hour, which is pretty nice considering it’s close to 100% profit (no tax or insurance deductions, etc). Say a piece takes you 10 hours to do, imagine its price was $200. Does that sound fair to you, considering the amount of work you put into it, and considering the costs of the material? If it doesn’t, increase your base wage until it does seem like a fair deal to you, both in terms of yourself not getting ripped off (selling just above cost, as you note) as well as if you imagine yourself being the purchaser.
If you start out with massive material costs, you may want to incorporate these into the price directly, then apply your wage on top. This can be a handy formula for doing commission works, as you never want to be put in a position where you pour hours into the job and you just barely cover your supply cost, especially if it’s a commission piece.
When it comes to art, and being your own boss in general, the guidelines are yours to make. There are some external groups which have a general recommended price structure, such as a graphic design guild. But you are under no obligation to abide by their rules unless you choose to do so.
Evaluate your pricing guidelines after a few pieces sell and adjust your rate as needed.
Blondesjon mentions a point I can elaborate on. My parents participated in a lot of craft shows, they made and sold stained glass lamps and glass pieces. They were the champions of bartering. They bartered a lamp for a set of beautiful unique paintings, for various spice and herb dip mixes that lasted years in our cupboard, for gifts to give to others, etc. They did have an advantage in that stained glass is a pretty high valued craft, then again it also costs a lot of money for the supplies and tons of hours of work per item. But in general, they made fair trades which satisfied both parties, and that’s really the best approach to take when trading or selling pieces in my mind.