Not sure yet, I'd like to test the setup first before tossing out prices. I hope to have a dual interface for both USB and PS/2 all combined including the 3 buttons. I assure you it will be well below what the mainstream USB PS/2 adaptor boards go for on the Happs trackballs.
Once i get a working prototype, i'll probably sell the interface seperately as well.....
Still waiting on parts, will update the thread as I get more information..
Also looking at clear trackball mods with rgb led lighting.