What's interesting about the P25 OTAP on the NX-5K radios is that on the page, it shows a license part number. Almost like it was set up to be a licensed per radio function. On my KPG-1 (a slightly out of date version) it's available for me to turn on.
Kenwood doesn't show it as a purchasable item on the price sheets, so I guess it's free/included with all of them.
On the NX-x00, NX-x10, NX-x20 radios we run, it's included and just a check box. I purchased the KPG-150 software when I purchased the rest of the system, and I'm really glad I did. I updated about 50 radios for our PD this week, and except for a few that were not active in the trunked system, they all updated without issue, all while I sat at my desk. I just had one of their guys make sure all the radios were turned on.