Just over a month ago The Guardian published a list of Brighton's top budget restaurants; I would second the recommendations for Iydea, Pompoko, and Pizzaface but I've not been to the rest -- or I have been but so long ago my experience is probably not relevant -- so I can't offer any insight there. Aside from La Choza. We're not very good at Mexican food here in Britain, no doubt due to the lack of a significant immigrant population, so that's something I've missed since moving back here. La Choza is quite good -- and not only by low British standards -- but it is a bit Guardiany, if you know what I mean.
About five minutes away, in a part of town that's a little rougher around the edges and not at all the sort of place that Guardian writers go, is Los Taquitos. The décor isn't anywhere near as precise and considered as that as La Choza's and the place does feel a little bit like a late-night burger or kebab shop -- a number of which can be found nearby -- but if you want a good burrito at a good price, Los Taquitos is the place to go.
This isn't a restaurant review -- not being able to try two-thirds of the menu means I'm not qualified for that -- but more of a recommendation. It's in a bit of an odd part of Brighton but if you like Mexican food, then you must make the effort to visit Los Taquitos, because it's ace.