Sponsor rates are around $100 - although this obviously depends on the quality of the template. For that the sponsor would obviously expect a sitewide link in the footer... Aviva and Alive are two directories that are known for providing sponorship...
I don't see how can everybody here can establish price for sponsored templates. It is like we talk about price for a car. Not every car cost the same. If you want to build backlinks by sponsoring template, several things should be considered: 1. Design feel and browsers compatibility 2. Changing just layout of default template isn't enough. There are lot's of elements that need to be styled, like forms on submit page, messages generated by the script etc. 3. Creator should know what kind of templates get used more than others. You need to have several different style templates created to know that. 4. Usability - design should have logical composition of elements that will help users to easy navigate and understand directory using it. 5. Mods-having preinstalled mods will also make difference-you shouldn't go too far with this too. 6. Template exposure is also important-more people knows about it more people can decide to use it. 7. Read me file should be written with full details for editing and customising template-this helps beginners to install and change it for their needs. So you can't compare template with few background images and css classes changed from default with some template made by all of this things on mind. Results need time to show the difference but you should predict your investment value before template is released, which is not easy. In conclusion, sponsoring 5 low quality templates(based on factors above) can cost you much more than sponsoring one quality and it can bring much much lower results.