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Of course they will have to pay more than the regular price. If the regular price is $500 for a template, then an exclusive one would cost around $5K. If the designer sell one copy for a $500, and he still has the right to sell the product, he could same this same design to other casino owners and make money. However, the exclusive one could cost you around $5K more or less. If you search for casino UI theme on google, you will find a lot of designs from dribble, and behance market. The casino owners can contact any of these designers and start working with them. I know a few to be honest. But I never like to be part of any deal. I don't have any benefits from these deals.
That will be the owner consideration to hire the designers to work privately or he just use while label for his casino. If he wants to press the budget and allocating for others, he might use the while label template but he will gives more things to satisfying his members. He will tells to his team (if he have) to works best they can and makes their members stay longer while playing many gambling games available. That will not be a problem if they use the similar design with other casinos but they must pay attention to other things such as services to satisfying their members. Sooner or later their members will know that the site is different than others although they have similar design.