Crush Lime Syrup is a refreshing choice. Hard to find, easy to enjoy. If you can find it you'd pay $3 per can. 2L jug. 2L of syrup makes approximately 33-39 cans of pop depending how you drink it sweet or light and bubbly.
Pops are a 5:1 ratio. The colder and more carbonated your water the less syrup you'll use. Start with aprx 150ml or 4 pumps for a 1L bottle and adjust +/- from there. Experiment with the number of pumps to find the amount that suits your taste.
Max out the box with 8L of syrup for the best value. Don't forget the syrup pump!