Welcome to the complex SPL economy! :D

You sort of need to know what you want from the economy. If you are after SPS airdrops, DEC is the most cost efficient, that's why the value shot up so much. Buying DEC is good in a sense as you will always need to use it. Buying cards and rentals are also measured in DEC, so it has the most utility currently.

Plots are valued so highly due to the potential and scarcity of them coupled with SPL insisting that there will be no more land releases period. Spending that kind of money on one plot seems a bit much for me and I can't pull the trigger on it...