Crush Lime Syrup 2L jug is a refreshing choice. Hard to find, easy to enjoy. If you can find it you'd pay $3 per can. 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 125ml or 3-4 pumps for a 1L bottle and adjust +/- from there. Experiment with the number of pumps to find the amount that suits your taste.