I'm also from Argentina and I can confirm that lens prices here are *very* different than in the US, but I must say that MercadoLibre is *especially* expensive. I've got much better deals through "alamaula" (a classifieds site also owned by ebay, but that works differently than ML).
For example, through alamaula I got:
A zenith camera with an helios 58 f2, a starblitz flash and a zenith camera bag, everything for 250 ARS (great deal)
A rikenon 50 1.7 for 200 ARS (good deal)
A K 135 2.5, an M 50 1.7 and an M 28 2.8, all 3 for 1500 ARS plus shipping (best f* deal evaaar!!!! esp. for the K135
On MercadoLibre, on the other hand, I have only purchased a Tak 50 f4, for 2000 ARS (yes, I paid 2 thousand ARS plus shipping)
And for the sake of completeness, on ebay, I've got an M 80-200 4.5 and a tak 35 3.5, for which I paid more money for shipping plus tax than for the lenses itselves (but cheaper overall than what I would have had to pay if bought through MercadoLibre).
Regarding the M 28 2.8, I believe it's a very capable lens, and I'm happy with it
(Although I've had no experience with any other 28s)