lead acid shouldn't be much at all. $30 to $40 maybe...
The Ducer is for open water and the UHD goes for 260 to 300 that I have seen online. Id recommend the GT10 Ice ducer for ease of use. I used an open water ducer on my garmin for a season, it worked but it was a pain because it doesn't self level like the ice dicer does so you will have to play around with it to get it to read correctly. I just got the GT10 and leave my open water ducer on my kayak.