As a Gondorian player, I'll echo the sentiments that it entirely depends on the army you collect.
The plastics are very cheap - far cheaper per model than any of
GW's other systems - but at the same time you tend to need a lot of them. Basically, each box contains 24 models of infantry, or 6 of cavalry, which is enough for three companies of either - and it's very rare to see a single block of troops made with fewer than 3 companies.
If you're collecting a horde army like goblins, you'll need an awful lot of kits to make up your forces. If, on the other hand, you're playing an elite force like elves, you can put together an army with relatively little expense, assuming you stick with the basic, plastic sets.
The forces of Minas Tirith are unusually flexible, allowing for large and resilient forces in the form of their core warriors, archers and knights, good quality archers in their rangers, and the best heavy cavalry in the game courtesy of the knights of Dol Amroth. They also have a variety of specialised units, many of whom can be very useful (the Grey Company, Axemen of Lossarnach, Clansmen of Lamedon, Men at Arms of Dol Amroth and Blackroot Vale archers spring to mind) - but almost all of which are exclusively metal figures (the army of the dead is an exception, but I'm not a fan. And the Grey Company can be part-proxied with standard rangers, at least).
All of which means this: you could field a fairly cheap 1500 point army by basing your forces around Knights of Dol Amroth. An example:
Dol Amroth knights: 3 formations (for flexibility), of 4 companies (any less and they die a bit too quickly), with a captain and banner bearer (important to ensure they can charge when required, and then succeed in charging), and you already have 855 points worth of army for the price of four boxes of plastic models.
A screen of warriors of Minas Tirith - say 2 formations of 5 companies - leaving you with 5 companies of archers (divided into two or three formations) as support* - and that's another 425 points done, for another five boxes of plastic.
This leaves you with 220 to spend on leaders - enough for Prince Imrahil and and Faramir (leaving 5 unused points), or for Aragorn or Boromir (leaving 20 unused points - enough to give one unit of your choice a musician, which will make them move slightly faster).
All in all, for the cost of nine boxes of plastic, and one or two heroes, you'll have a decent 1500 point force - not only that, but one that'll look pretty damned impressive on the field, being made up of 80 heavy infantry models, 40 archers, 24 heavy cavalry and one or two heroes.
You could easily spice things up by switching out some of the models for rangers, knights of Minas Tirith, or the army of the dead. Things get more expensive if you start looking at the more specialised units (and god forbid you start an entire army based on the fiefdoms - that would be a very, very expensive endeavour), but a small formation or two won't break the bank.
My own army is an infantry-based force, so a deal more expensive - not least because I eschewed archers of Minas Tirith entirely in favour of rangers, meaning a lot of wasted plastic. I currently field 96 warriors, 72 rangers, 40 grey company and 8 knights of Dol Amroth, plus Aragorn and Faramir, all at 1500 points. It cost a fair bit to accrue, but mostly because of my reliance on relatively cheap massed infantry, my refusal to use my archers (I have 48 sitting around), my use of the Grey Company (of which 19 figures are metals, including Halbarad), of the rangers of Ithilien (meaning another two metal heroes to lead them, Damrod and Madril), and of two epic heroes on top. Even with all that, I can think of no WFB or
40k army that would be so large, effective and varied at the same price.
I'm a big fan, in case you couldn't tell
*you get one company of archers with every two companies of warriors in the box of plastics - annoying, if you don't want archers. Even more annoying if you
only want archers. Fortunately, both are useful core units.