Again Tobus, there may be more than one factor here.

Anyone can design a Tartan. The on-line design programs will automatically create the design (Warp and Weft) from the colors you choose and the number of yarns of each color.

But - Not every Tartan design will make an attractive kilt and some do not work for pleating at all. There are some totally hideous designs out there. All you have to do is have a look though the thousands of designs at places like ScotWeb's Tartan designer program.

So you now need a kiltmaker to look at the design with a experienced makers eye. Will the design look good when pleated? Will it look too busy or too plain when made up into a kilt? Will the pleats, and their width, look the way the original designer intended? Will the design create problems for the kiltmaker when they taper the pleats? Or - is the Sett size appropriate for a kilt kilt? etc. A good quote from one kiltmaker is appropriate - "It's not the cost of the fabric, the thread and to keep my electricity bill paid that you are paying for. - It's the years I spent learning and perfecting my craft that you are paying for."

Then the thread count is give to the weaver and they may need to make further slight alterations to the thread count to center the design on the loom or to get one stripe a particular distance from a selvedge edge or to create a given sett size. This is one of the reasons for the rule of thumb that says a Tartan does not have a stripe of just one thread. That design cannot be halved by the weaver if needed.

The designing on one of the on-line programs is usually free and anyone can try their hand at designing a Tartan.
Where the cost comes in is having a kiltmaker design a Tartan that will look good when made up.
The weavers part is usually included in the cost of the finished fabric. There would be some cases where the weaver may ask for a slight up charge of course but those are fairly rare.

So, someone who asks for a professional to create the design may be charged. $350.00 to $450.00 is about the average fee for a professional design.
Then if the person wishes to list their design on The Scottish Register of Tartan there is a fee. Currently that fee is £40.00 and there are some additional fees as applicable.
Then you approach a weaver to have the design woven. I have already given examples of some of those costs.
Add shipping taxes and duties on the fabric depending on where you live.
If you wish to protect your design with Copyright there may be further costs depending on where the Copyright is awarded. A Copyright in only one country will be significantly less than an international Copyright which usually requires a Copyright lawyer with additional costs.

Then, and only then are we talking about the cost to have a kilt made from your woven fabric from your listed design. (See the quote above)

The ultimate and final cost of a kilt in a custom design has ended more than one marriage. And saved a few others.