If you want a fairly decent quality your not going to get much more affordable that what your already feeding unless you want to drop quality. Another thing you have to consider is though price of 1 bag of food may be higher than what your currently feeding, the bag will also last longer therefore in the long run you will end up saving money or at very least no difference.
Heres a prime example, i used to feed TOTW Wetlands with both my dogs. 1 bag would last 2 weeks, i switched them several months ago to Orijen 6 Fish/Regional Red. 1 bag of either/or will last me 3 - 3 1/2 weeks.
1 bag of TOTW - $44
1 bag of Orijen - $80
More than double however i calculated that in 1 year i am roughly saving $30 feeding Orijen vs TOTW.
If you want something better quality but still close to your price range..
Taste Of The Wild
Canidae
Natures Variety
If you want something cheaper:
Diamond Naturals
Though personally i don't care for Diamond due to the cheap ingredients they put in their food and you will end up feeding ALOT more than what your feeding now to get the same intake to keep the weight on. Oh and if you do go that route, like all cheap foods, he will be pooping ALOT