I just use the regular cola flavor with cherry syrup flavoring. Works great for me, but I can't attest to how cheap it is.

The cola comes out to about 0.55 a liter. amazon link

and the cherry syrup comes out to about 0.34 an Oz amazon link

So all told probably not actually cheaper that what you have now, but I like being able to adjust how much cherry there is(or skip the cola completely sometimes)