First, I don't think adding in the flight box is valid in the comparison - it is needed either way. So that makes it $390 versus $480 which is nowalmost a 25% difference. Second, as you point out building requires tools that you don't typically have with an RTF. That makes it even more.
But what you are really trying to get to is that building is a lot of fun - to some people. I love to build and have done several. But I also love to fly and when I crashed my kit-built PT40 I got back in the air quick with an ARF. Now I have been out of it for about 10 years and have a limited flying season in a limited flying area so I went with a smal electric RTF to get back up. But this winter it's another kit I put away far too long and am having a ball .

In summary, i really wish people wold stop brining up this kit versus RFT versus ARF. It is not useful and only creates animosity.
And yes that was from someone who likes to create animosity