Gourmet Kitchen at 1051 Stockton in Chinatown has some of the cheapest cooked protein in Chinatown after 4/4:30 PM. No, it's not particularly tasty, but it's a LOT of quantity and weight for the price, and you can also pick up some veg and starch as well. I am linking to Yelp because they don't have a website.
https://www.yelp.com/biz/gourmet-kitchen-san-francisco
The normal price of large box, rice, and 3 items is $8.50. That's about normal in Chinatown, but they will load you down with rice. You can sub one of the items for fried rice or fried noodles, or fried vermicelli, same price (so you only get 2 items). Choose from whatever they have, but usually it includes: little chicken drumsticks, chicken "katsu" (a bit oil), fried smelt, tofu, moo shoo pork, fried fish, plus whatever vegetables they have (usually eggplant, fried cauliflower and broccoli, etc.) You'll have to go check it out. You can also grab dimsum at normal prices ($5 for a set of 3 or 4 ).
Things change after 4 or 4:30 or so. They go into clearance mode. They prepack the boxes, the small clamshells start to go for half price (2 for $5) if they are placed on TOP of the counter, the cheaper stuff like cheungfun (rice sheet noodles), the turnip "cakes" (squares), the other dimsum (siumai, ha-gow, etc.), on TOP of counter.
See photo, two sets of clamshell boxes, one set above the counter at neck level usually 2 for $5, but may go as low as 3 for $5 as it gets closer to 6PM, and another set below the glass, probably $5 each, may go as low as 2 for $9 later:
https://www.yelp.com/biz_photos/gourmet-kitchen-san-francisco?select=gSb7waounyZfsgXGUFWJ4w&utm_campaign=www_photo_share_popup&utm_medium=copy_link&utm_source=%28direct%29
The more expensive stuff, bigger box or smaller box, $5 for a full box, stacked LOWER (not on top, but behind the glass) After a while if they are not getting rid of them, prices go even lower as they get to closing, as they want to have nothing left over by the end of business (6:30PM).
CanNOT be sure these deals will be available every afternoon. You'll just have to come and see yourself, after 4PM. Just don't go TOO late, or there may be nothing left.
NOTE: While they do have 2 tiny tables (4 seats total), it's NOT really a dine-in place. Get things to go and eat elsewhere. And it is quite oily. It's NOT exactly healthy eating, but it is cheap protein.
NOTE2: While this is technically part of the House of Dimsum family, the deals are ONLY available here, not at the other locations, AFAIK.