I would cut back the amount you feed the two large dogs and that will save you some. Three bags a month is a lot for two dogs. One bag of Natural Balance (30 lbs.) feeds two of mine for a month. The other dog gets a bit more, but still only 4 cups a day and that is for him to gain weight.
I would try to keep the beagle on as much of the Blue as you can do, unless he has had the Purina before and did well on it. Mix it now so it isn't a sudden change.
If you need to stretch it more I would add rice, veggies, eggs, meats.
You could also look at expiration dates on the bags. If they are close to expiring the store may give you a discount. Our store starts marking food down at 90 days from expiration. When it gets to 30 days the price really drops. Maybe go in a day the Blue representative is in the store and see if you could get some samples.
Those are just the things I would do in the same situation. I would probably stay with Purina if I needed to add a different food unless it was just small amounts.
I do believe that on the food analysis sites Ole Roy ranks higher than Beneful.