Okay makan-wise, eating-out in restaurants can get pretty expensive (as everywhere in the world actually haha) but it really depends on your priorities towards food. I rarely save on food, and was almost bound outskirts to visit that Michelin-3-starred lady chef, reservations et al, until something came up. My personal adage is: You won't get many chances of savoring food in a foreign city so why scrimp.
Avoid the touristy places, the ones with cheap buffets, common traveling sense I suppose. I know we tend to get homesick after awhile and want something familiar but have heard from hostel mates that the ristorante chinos (of which I've been asked several times by locals if I worked at one
) here are pretty crappy. Also note the eating culture here is totally different, meals take a looooong time (a lot of talking and drinking involved) and dinner starts at 9pm. Which means most restaurants open exactly at 9; there's a queue right up front, everything's ready, the waitress is fiddling with her apron, maitre'd doing a lame this-spoon-looks-dirty-why-don't-i-polish-it-to-avoid-hungry-customers-stares, and they still won't open the doors at 8.55pm.
If you're going cheap, or travelling light and fast, you can make your own sandwiches. Grab bread and cheese and then try the local cuts of meat: the jamons (hams). Supermarkets will have them but best still are the butcheries, the whole place is filled with hanging meats from sausages to whole cuts, reminds ya of the chinatown pre-cny waxed meat stalls. Mind you most are really done the way it should be: cured air-dried by hanging up in a cellar (or warehouse as a sign of modern times) so if you really think about it, they're umm raw. So goes the sausages too. Local fruit stores and bakeries can be found parallel to Passeig de Gracia in Carrer de Avinyo (I think that's what it's called), good to get things you don't normally see and are able to eat while walking. I remember constantly having these hybrids between apples and pears in my bag for a quick munch, cold winter = hungry me. FYI, these are all great photo places
The bakeries especially display the sweet stuff out at the window: think faces pressed against old-fashioned glass pane peering into the shop at the stacks and stacks of cakes, macaroons, cupcakes, pralines, etc.
If I really had to recommend anything then yup, tapas bars. The traditional ones at least, polished mahogany bar, cervesa on tap, and the salads in small trays on the counter. Vaso de Oro, this place off Port Vell, one of the first I ventured in, had the rustic authentic feel; also because it was pretty darn secluded.
But for something easily accessible: Cervesseria Catalana and their other branch Ciudad Condal (rich wood + green paint decor looks the same, can't miss the association). The first is off Rambla de Catalunya (parallel to Passeig de Gracia) somewhere, one of those turns to the left if you're walking from La Rambla/Placa Catalunya, not too far, just the 2nd or 3rd storefront once you turn. The second is right at the start of Rambla de Catalunya, corner shop, diagonal from Placa Catalunya itself. This is where I spent most my meals, though I stay put, you'll notice that locals do the bar-hopping thing. One drink, a few plates of tapas shared then off to the next one. Anyway note that both establishments get really crowded, standing room only and even the bar/counter is filled. On average I spend about 40E on dinner and I don't drink, but you'd do better if you avoid the grilled seafood (again la plancha or al ajilo - with garlic). Things to try would be their chipirons, baby squids, like the deep-fried ones you sometimes get in a chinese dinner cold dish, but these are grilled; cuttlefish and razor/bamboo clams. Best item they have is the solomillo, a bite-sized steak on a toothpick, served with these small grilled green chillis with coarse salt sprinkled all over. These are staples in most tapas bars, btw; interesting to try the different takes on the same items heh.
And don't forget La Boqueria...