I've followed this thread, trying to come up with some useful advice. To be honest I don't really have any answers, only a few more twists on the questions.
You seem to be very serious and realistic regarding the issues and potential costs involved, and in wanting to reach a balance of fair compensation for both yourself and the artist. But as has been covered by previous posts, for many reasons this isn't easy.
One reasonable way to achieve a balance would be to share the risks and rewards. i.e. pay an initial amount to the artists as an advance against royalties. If the project doesn't do as well as hoped, he or she will receive some compensation. If on the other hand the project becomes the next Thoth, then they can share in that success also.
Either way at some point, irrespective of how its eventually compensated, a value for the work has to be defined. Someone suggested in an earlier post (which I can't find now) that you consider looking for a comic book, or digital artist, because they work quicker. This seems to equate that therefore it will be cheaper. To a degree this is true, the time verses value is one obvious connection. However to start with I have to say the reference to digital art being quicker is misleading and once again demonstrates the misconceptions of this media. Its NOT the media, its the style that determines the time and this is one aspect that you haven't referred to in your posts. You clearly have a defined idea of the concept and content of your future deck, possibly even the layout color scheme etc. But do you have an idea in your head of the style? because that will probably be the most important factor in determining how much work (time, effort and cost) any suitable artist would charge.
I don't wish to suggest one is better than another, but for the sake of comparison, a simple line drawing and flat color like approach of say a Hello Kitty would presumably represent far less time than highly detailed images, irrespective of weather it was done digitally or oil on canvas.
Having a clearer idea of what you envisage will narrow the search for potential artists.
If indeed the style can be achieved by a digital means, that will also influence the legal issue of who owns the original, since by default the original is not a tangible entity in the traditional sense, from which "copies" are made.
Other considerations that might be worth considering, is that of having different rights for different uses. For example you value your original ideas and wish to maintain the rights (control) over your concept in tarot terms. But possibly the artist might keep control of the use of individual images. This sound complicated but essentially its this. The publisher has the rights to my images as a collective entity i.e. the 78 images which as a set constitute a tarot product. Were I to produce a set of those images reproduced on cloth, or whatever, the publisher might argue that they conflict (in its similarity) with the deck itself. Nevertheless, I still maintain the rights to each individual image. So I can sell prints of those individual images, license one or more to third parties for reproduction on merchandise, rearrange elements to create a themed montage etc etc.
This kind of thing might sweeten the appeal to an artist without it necessarily infringing on your core concept, which may be more of structual issue related to the full deck.... if that makes sense.
Nevertheless I think you'll find that the more accomplished the artist the less likely they are to blindly follow your direction. Personally I would find that so frustrating, all that work simply to follow instructions???? It would drive me nuts. But if there is any input from the artist in terms of feedback, suggestions, changes and adaptations, while that could be a good thing, it also may muddy the waters. It would not then be a clean division between your ideas, and a paid effort of a contracted artist.
Last but not least, and this is where I run the risk of putting my foot in it. No matter how good you consider your idea to be, conceptually, symbolically etc etc. Ultimately any future publisher and customers is going to be primarily influenced by the imagery. Both in deciding wether to accept the proposal in the first place and then the actual sales. The structure and underlying concepts are secondary (not in tarot importance), but certainly in determining the initial and early business viability. So the imagery is far more than simply the manifestation of your "baby".
Lots to ponder here.... Good luck in your search, and I hope all goes well.