In general, prices for theme design work will range depending on a number of factors. If you don’t mind the designer releasing the theme on their site for others to purchase, then you can expect anywhere from $20US to $50US for your theme. If the theme must be a one of a kind theme that only you use, you could expect to multipy the fee by at least 10x.
Also, there is the option of finding a PSD based webiste design that you can get permission to “port” to XOOPS. Essentially, just look for a free website template that has the design you want and write the author asking if you are allowed to modify the template for use on your site. You may even want to inquire as to whether or not you can re-release it to the community. Make sure you get written permission to do this as the original designer has full copy rights to all images in the design, even if it is a “free” template.
If you don’t feel confident “porting” a static website template to XOOPS, there are some who specialize in this an would do it for a nominal fee, usually $50-$75US depending on the complexity of the template. Some have to...