Yes the fluid is cheap, and if their is water in it, the Transfer case repair is not cheap.

BTW - you need GL-4 compatible fluid. It can be difficult to find. It is not GL-5 is "newer" "better" thing. You need GL-4 for the transfer case and manual transmissions. GL-5 (or higher - GL-5+) for the front and rear differentials. If you have a limited slip diffy you need to ensure that the fluid is rated for a limited slip diffy or put in the additive for limited slip.

'90 Montero XLS (Sold, sadly)
'95 Montero LS 160k
'03 Montero Limited 30k