hmmm... not many responses since Tuesday.
Sadly, it may only be worth the price of the lot. Unless you want to bank on a proposed design & decorating event (link below) or lobby for changes to the Luna GZ.
Aside for the castles and keeps, it seems that unless a plot is within a GZ or close enough to allow vendor shopping from within a GZ the resale value of the plot will not be great. Strange for Siege, but it seems to be true for most of the population based on the New Mag house bidding (other link below).
Luna House: Self appointed Governor of Luna has a design & decoration competition for houses located within the city walls of Luna - - with various Luna opinions
http://community.stratics.com/threads/the-governance-of-luna.328892/
New Mag GZ House: Last know bid was 60mil and was to end on the 19th of Dec, but still has same owner and the deal has not closed.
http://community.stratics.com/threads/new-mag-house-up-for-sale.328067/
Or... you can hang on to the plot and lobby for changes. My two cents, the inner walls of Luna containing house plots should be included within the GZ. Most new players to Siege probably think Luna is the going to be the place to shop for goods then get discouraged when they see very few goods and very people in Luna. Granted, there is the New Guild and New Mag, but why not have more options available to boost the shard and help the newer folks make the transition to Siege? Just a thought....