Well... The value for these is right now based on the fact there is a limited supply and MA promises (6 or 7 years old now) to give plots commercial utility in the future.
So it's hard to put an actual monetary value of something like this with only vague promises from MA.
I can tell you that the last few Medium plots have sold for around 10k, though they actually cost more than 20k to obtain initially when they were released. I guess the holders got tired of waiting and gave up.
Small plots have sold for anywhere from 1800 to over 4k, it's hard to find them for sale so I think anywhere from 2,5k to 4,5k is fair considering apartments on other planets go for at least 2k each and they are just basically a fancy secondary storage whereas small plots can have something built on them and gardening and future commercial utility.
It's mainly a speculative asset at this time though.