I bought an Argentine mattress which is pretty good(similar to the brand name ones in us), it's not foam though, Forgot the name(start with P), but their ad is all over the town.Probably the best in Argentina, each spring is in its own silk tube so it only goes up and down straight, very tight and firm. Many local stores in Recoleta carry it.
I tried the foam one in bsAs, wasn't that great. The foam was not thick enough for me. I have foam mattress in US, this is not like that.
Argentines make good mattress, go to a bigger furniture store and talk to someone, usually they have to order and deliver. 600- 1000 us$ is common.
So 5000-10000 pesos for decent ones.
These foam ones are not nearly close to the the ones you can get in US. the thickness should be 6 inches, these look like only
2-3 inches. At 3600 pesos, it's half the US price.
I think there was a bed store in Rodriguez Pena y Guido (or like 1 block around there). Never been inside personally, but if you are in the area you might take a look. Also, they always had advertisements on the windows for discounts when you buy with a credit card of a local bank. Not sure if this is still common practice, but if so, you might save 20-30% by using a local credit card.