I think my dealer was around $600 for the Kioti branded third function. However, I'd caution you to afford yourself a bit more time than you might think during the install. I had challenges with the instructions. They were obviously written by someone for whom English is a second language, and they weren't simplistic. The directions referenced the same part by two different references (1 and N may be the same part), and you had to have two different references side by side to keep them straight.
I wouldn't discount an aftermarket third function, if you're looking for a quality part.