That's actually not a bad comparison really. Groot is 38.9cm tall, so 3cm shorter than Ultron. He also has a switch out head, and multiple hands...and a beautiful sculpt as well! That's all for $199.99.
Ultron has the swap out palms, and the light up feature...and unless I'm missing something, that's it. So the light up feature, and the light up base are the two things that set Ultron apart...as well as the extra 3cm (which is just shy of an inch).
It was said earlier that the HK conversion would see Ultron come in at $300 US without any markup. I figure he'll land between $324.99-$349.99 somewhere...but now the Groot comparison has definitely made that seem really pricey. I think he'd be easier to swallow at $279.99 or in and around there (which bumps him up $80 from Groot for the 3cm difference, as well as the light up features -- though Groot did have the alternate HS which can't be ignored.) Again, maybe I'm missing something though! Oh, I guess there's the Avengers tax as well