Like everything else in the base, I would say its cumulative..
Would be suprised if it wasn't...
But have yet to find this out, as we don't have that much Pre yet :P
I suggest you go to the test server, register a SG and get the 36 mill there, and go test it 
lol very good point, will go have a look on test and respend my 36mil 
They are not. If you had paid 3 mill and then want to buy one with a price of 8 mill you would have to pay 8 mill.
But obviously... if you pay 3mil, then want to upgrade to the 8mil plot, you're gonna sell the 3mil plot... which means you only need to remaining 5mil.
So its not cumulative... but it is at the same time...
You don't get back the price of a plot. If you buy a plot for 3mil then buy a plot for 8mil you just spent 11mil. At least thats how it worked in the beta. Makes plot upgrades bloody expensive.
From my experience on the test server, you can sell your plot, o to downsize your base you sell the plot you've got and replace it with a smaller one, to upscale the shebang, you sell your little plot, buy the big plot and only pay the difference.
Anyone know if the cost of plot upgrades is cummalitive or not?
Eg there is a 3mil and an 8mil plot. If I buy the 3mil one, do I pay an additional 5mil to upgrade again? or is it still 8 mil?