You could buy the regular sized Nutro kibble, put some in a Ziploc bag and crush it up a bit with a rolling pin. Or use a blender or food processor.
As far as other more affordable brands . . . I don't know off the top of my head how much Nutro costs. But you might look at Authority, which is Petsmart's store brand. The ingredients seem pretty decent. Simply Nourish is also a Petsmart brand whose ingredients look pretty good. It's probably considered a step up from Authority, and I believe it costs a bit more. Since they're Petsmart's own brands I'm guessing they may put them on sale frequently.