I do art commissions and I should keep the above point regarding cost in mind.. I plan on redoing my commission prices eventuallyA common mistake of many makers and artisans is to undercut themselves, thinking "it's just a hobby" and so I'm glad you asked the question.
There's a variety of different formulas available. I suggest seeing what they come up with, and that will give you a range to work with.
Another one I like is to blend
A) 3x cost of materials
B) cost of materials + what you consider to be the value of your work to produce an item (e.g. 1.0 hour x $15).
A is a common pricing strategy for items that don't require individual creation (e.g. postcards and stickers that you had made from a drawing you made)
B is basically the baseline, and in @Atrapa Almas' example, is a variation that also builds in a margin.
Having a margin is always good, as that provides you with the reward for selling it - basically, you get earnings for both the creation and for the selling. And you'd want to be able to give a modest discount for multiple purchases, or repeat buyers.
Where you are considering the possibility of consignment, having a higher margin is helpful, since 25-50% commission to the store is quite common, and you want to ensure that you're still getting a fair amount.
The other point I want to cover is shipping. Generally, I'd recommend actual postage+packaging costs and up to $1 more to cover incidentals. People that buy enough online get familiar with costs, and are turned off when a seller has an obviously inflated "shipping and handling."
The packaging is where you get to show some additional creativity - a thank you note or message written on the back of a business card, perhaps an organza bag or other simple but elegant packaging, and maybe some fun stickers with your logo or other art to adorn the envelope with. You want to show your pride in your creation, and for the buyer to get excited when they get it - it's not just another widget that someone got from Amazon in yet another generic brown envelope, but something you lovingly made - for them to enjoy and cherish.