step 1: ditch the purina, your payig alot of money for a so=so food, and your dogs are showing you that by eating so much...
if you have a costco id suggest kirkland...if you have a
tsc, diamond naturals or 4 health. better food and youll feed less of it overall, i think its also cheaper than purina for a bigger bag.
step 2, do NOT bulk it out with "cheap food" youll simply find yourself burried in poop and upset tummies...
instead if you find you need to bulk up the food, buy ground hamburger or chicken (big packs are often chepaer per lb, and look for meats on or very close to expirey (they typiclaly have addiotnal off) and a bag of whole grain rice (white will do if money is realy tight.
(now i would personally skip the rice and feed the meat raw but if your using it to supliment kibble...cook it as raw digests quickler than kibble and it can upset the system) cook the meat (boil, broil or brown and drain in a pan, no addes seasonings) boil up the rice and tada, a much healthier (and beliv eit or not cheaper) way to bulk out your dogs food.
eggs and cottage cheese are also goo high protine ways to supliment.
belive me i know what its like when money gets tight...and making your own meat and rice to add a little bulk should save you a bit of money if your willing to shop sales, also place adds for freezer burnt meats on your loacal craigslist and free cycles, dogs dnt mind freezer burnt!
i do think though that 7 cups a day of food is a little much, and you wont be feeding 1/2 that if you could get them on a better food with less fillers...