You suck at teh Japan.
Pay attention to which days of the week food is cheap. Hirakata is a student town, so if you have a bike, ride away from the station and keep your eyes peeled for a grocery store. The one in the basement near the station is a bit pricey but it does have sales, you just need to pay attention to when they are.
You should be paying about 68yen per 100g of chicken breast, tops. I usually manage to find chicken breasts for 48yen/100g, so any price inbetween would leave you paying about 160yen for a decent sized serving.
Tuna in water. Look for the Top Value brand. It is a nationally sold brand available in many grocery stores. You can usually buy 4 cans (with 16g of protein per can) for 300-400yen. So again, you'd be looking at about 150-200yen for a serving (30g) of protein.
Milk is cheap, you can by low-fat for cheaper than the 3.8% full fat milk. I usually buy 1% for about 100yen per jug.
Protein supplements: buy them from bodybuilding.com They have decent prices and don't fuck around with shipping. Other places might be cheaper, but bb.com you just pay with a credit card and they ship it, other websites gave me hassles, so fuck em. When you consider that the quality of whey protein in Japan is questionable and have many fillers (bulking and cutting carbs?), and expensive, the cost is actually cheaper to buy a 5kg bag of ON Whey from bb.com than to buy the same amount from a no-name j-brand.
Veggies: you can buy a bag of frozen mixed veggies for 200-300yen, tops. Half a bag is enough for one meal. So for 200yen for chicken, plus 150yen for veggies you have a decent healthy meal. Right now is an in-between season so not much is cheap. Come summer though produce should be cheaper.
Brown rice: buy a big ass bag from Don Quioxte and you'll be set.
Oh, you might know this already but do you meat shopping at night when the meat is often discounted anywhere from 10-50% off.
Eggs: cheap.
I spend more money on groceries than most people, but if I cook most of my meals then it breaks about even compared to eating out all the time. But I eat about 4 times more protein than the average person.