This is a very tough question to answer. If you aren't in business and do this as a hobbyist, and it's for a friend or relative, just covering your costs can seem inappropriate. But, even in this setting, it's time out of your life.
Beyond that philosophy, if you are doing this for a living, your costs can be done as a line item to show your clients. Even general contractors do it that way. They have a line for labor, materials, overhead, and profit. That seems to be a legitimate way of presenting costs.
Then, you have the relative value of the item you are making. In other words, what does that item sell for in stores, or what could they buy it for if they shop it around. Even with that, the items they look for will not be "custom made". Your work is to order, of specific sizes, and design. Nothing downgrades a piece of custom furniture other than the piece itself. That being if it is made to a certain standard and has that "custom" touch.
If the work is laden with repairs from mistakes in fabrication or finishing, that would have to be addressed, and that only becomes a bargaining point if it is allowed, either on your part (for mentioning it), or their part (to get a cheaper price).
You have an investment in tools and equipment, along with the talent to produce a piece of furniture. If the piece turns out real nice, what difference does it make if it's your first or 15th piece. I went through the same questions when I started out. Not knowing what to charge, and feeling guilty.
There are a few ways to figure what to charge. The 3X method may cover your costs and put some money in your pocket. You said you have 30 hours and $250 invested. Time out of your life isn't based on an hourly charge. But, since this is an exchange of furniture for money, you may figure out what the 30hrs are worth. Labor at $10/hour is far from what a skilled cabinetmaker is worth. But, when you get more experience, you will likely have a per foot charge, which will get very close to what's fair. By that time, you'll know from your paperwork, figuring and fabricating, how much lumber, plywood, and hardware it will take for how many cabinets/doors.
Having a price per foot will enable you to estimate projects quickly, as you will be within the parameters of a profitable job. I have a price per foot for base cabinets, upper cabinets, counter tops, back splashes, doors and drawers. It is a variable method for the type of cabinet (mica laminate, or finished wood).
So, pricing can be done a few ways. Trying to beat someone else's price may only put you in the hole. Not only for time and materials, but psychologically. Just knowing that you could have easily collected another hundred or two, or thinking you are really working for nothing. Even in this case, with the $250, as a fixed amount, and 30 hours (@$10/hr) you are talkin' $1050. That doesn't cover the cost of a place to do the work, utilities, and likely the consumables used on the job.
I don't know if I helped you or not. You have to be satisfied with your pricing structure if you think it's fair. Just remember that it's easier to come down in price than go up.