The real problem with this idea is that then you end up with (potentially) land barons who own all the good plots. At present there are people who have a lot of subs. I have 3 of my own. Whilst I do have a lot of characters that I can play, generally I play one or two of 5 main ones.
So, taking the plot ownership at $5 per (admittedly the cheapest example) I could spend my $45 per month on one gifted sub ($15) for my 5 characters and then 6 x plot subs ($30) and own 7 plots of land.
It would not take long before all the best plots are snapped up. Additionally, there are people in game who have far more money that most other people and can easily outbid others in an auction.
There are a lot of people who do not have lairs/plots now, and they are the ones who are less likely to have a load of ingame cash (Unitarians included, who have not had the last 12 months or so of building cash reserves, or at least not with their fully developed characters). Making plots cheaper (in RL terms) is only going to make things worse for those people.