All you can eat?
What, are you Kevin Smith?
Personally, I can barely eat half a bife in most parrilla's, so the idea of needing more is kinda hard for me to understand.
Plus, the cheaper the parrilla, the gristlier and fattier the bife- but I guess some people believe in quantity over quality.
Anyway, for cheap, but perfectly edible parrilla in quantities beyond my meager ability to consume, I recommend La Gran Hollywood, on Bonpland y Guatemala
http://www.lagranhollywood.com.ar/index.html
Its simple, funky, and local. No hint of the fancy tourists that frequent La Cabrera. But plates of fritas about as big as cafeteria trays, bife for $25 pesos or less.
Most of the really cheap places are NOT in Palermo- why would they be? The rents are higher, and the average clientele in Palermo Soho is far more concerned with being slim and seen than all you can eat.
A bit farther away, in Almagro, but not THAT far, maybe ten blocks from Palermo, is ALE ALE, with all you can eat salad bar, and huge portions of fixed price meal specials with multiple courses- its the kind of place where cheap chowhounds go to shovel it down, but its pretty darn good, if you ask me- Estado de Isreal just a few blocks south of Cordoba. Parrilla, of course, but another 50 pages or so of menu items too...
http://www.guiaoleo.com.ar/detail.php?ID=65