I am not a dealer but I see on multiple web sites they are $349. On your side of the planet I would contact Tokyo-Offroad as they appear to be a dealer.
https://www.tokyooffroad.com/product-category/stic/ They also have some great videos about the STIC.
It sounds like you have a great running bike! It is hard for me to say how much it will help your bike. I can say it was well worth it for all 5 bikes I put them in. I have one more to install in my other son's 2004 DE300. That 300 is also a great running bike so I will report back after I install it. It will be a few weeks before I get back in town to play with it.