This is because when the last member of the household dies they take the furniture with them. The lot 'appears' to be furnished but it really isn't, hence the low worth of the lot. Even using the free real estate money cheat you will only get the move in unfurnished option.
If you go into build/buy mode, save the lot then place it back down from you gallery, it will be furnished.
One way to prevent this is to add a young adult sim or family into the lot before the last elder dies (which is what I do).
It is really annoying this happens and I honestly cannot see this ever being fixed.
If you don't mind using mods, MC Comand Centre happily moves sims into unfurnished lots.
