A 5lb bag of food for one bird being fed a sensible portion 2X a day will last about 6 weeks in my experience. If you're overfeeding (which most people do, filling up dishes WAY more than their bird really needs), figure a bag a month. My avian vet recommended Volkmans seed mixes, though I'm unclear what specific mix she'd suggest for a small parrot like an IRN. Pellets are a processed 'food' and were originally developed for people too lazy to feed their birds an appropriate diet or birds who refused to eat an appropriate diet according to my vet. She also says that many brands contain GMO corn and soy along with all kinds of dyes and preservatives many parrots are sensitive to and can cause them discomfort or more serious health issues. Plus, nothing in them is really part of a parrots natural diet. In nature, these birds eat fruit, nuts and seeds, not pellets made of GMO grains/beans and miscellaneous laboratory chemicals. She doesn't recommend them unless the bird won't eat healthy food, though you should speak to your own avian vet about the matter and see what they have to say. I happen to agree with my vet on her view of pellets but as you will undoubtedly see in subsequent responses, many people will fervently defend pellets and many avian vets would too. It's a choice you'll need to research and draw your own conclusions on.
Speaking of appropriate diets for parrots, you also need to factor in the cost of fresh produce, which your bird will need to eat a variety of daily. If you eat lots of fresh produce yourself, I wouldn't bother factoring in extra cost. A bird only eats small pieces, so just offer some whenever you eat some (bird safe varieties, of course

). Otherwise, you will have to buy it special as parrots need to eat a variety of fruit and veggies every day and that could add up if you don't already buy them in abundance. Canned is a firm NO, frozen isn't recommended and dehydrated is only ok if unsulfered and preservative dye free (basically, you would need to buy a dehydrator and do it at home). Fresh, is of course, best. Some people make chop, as did I for years, but my vet again recommended against it because it really isn't food in a natural form.
Minor expenditure: Your bird will also eat *unsalted* nuts, though those are more 'treats' but still a potential dietary cost you'll want to factor in. My vet recommends a couple nuts a day for my amazon, to keep his gizzard in good working condition

You may want to consult your vet for an appropriate number of nuts for a bird the size of an IRN. Apparently, nuts are good for gizzards because they are hard and give the gizzard something to grind up (which is what it does). Too much soft food makes for a weak gizzard and makes them more prone to digestive issues.