I doubt that you'll ever find the true cost anywhere. I believe that invoice on cars is a manufactured number and not really the dealer cost.
Go to the Kubota site and do a build my Kubota. That would be your starting point. MSRP is listed..From there I would just start making phone calls. Someone around you will outbid everyone else. Happened every time in my case. Some dealers think they're sitting on gold and won't budge, others will be more than happy to work with you. Sometimes volume is better than hitting home runs.
Mine listed around 18k, but since it was a leftover, I got a fairly good discount.
Don't forget to try Barlow's as well. I didn't end up buying there, but they were unbelievable to work with. Kudos to them.