Is it expensive to eat out in centro ?

I’m looking to visit in mid February for 2-3 weeks primarily training Muay Thai in a gym in the centre. Ive a good deal for a studio apartment where I’d make my breakfast and some times lunch but ideally I’d like to eat out every night. I’ve heard the prices can be insane around the hotel zone but I’ve no interest in there anyway. I was wondering if it’s possible to eat out for cheap in the town. I would much rather eat where the locals go anyway regardless of price

Also is it possible to go to the beach without having to go through one of the clubs ?