And remember, they don't want to invalidate the existing homes. There still needs to be a point to buying Proudspire or Honeyside, or Breezehome, and since they won't be customizable like the new lots, then the point would have to be "more affordable housing"
Cheaper housing wouldn't validate the houses. I can just save my money (seeing as how food and shelter are not vital to my character's survival). What does validate the houses in the cities would be the merchants and nearby locations of many of the game's quest givers.
And it would make absolutely no sense for a plot of land to cost 100k. First off, houses in the cities should cost several times more than that plot of land because it is in a protected community with nearby merchants (aka location). Second, those plots of land in the cities come with partially furnished houses in them. Third, you are in the middle of a warzone. Fourth, the wilderness in Skyrim is hella dangerous.
There is literally no reason why a plot of land should cost more than the house in Solitude. Making it so would certainly help to break immersion (which would defeat the whole purpose of buying land and building your own house).