If neroli oil is so expensive, why add it at all? There are so many different oils in it, is a single one of them that important for the whole taste?
It is fun, but very very time consuming because you need to stretch ~10ml into 7gallons of water until you can know if you are on the right track.
I have also experimented with adding vanillin, which I believe commercial Coca-Cola contains, but which is curiously absent from the OpenCola recipe. It's great, though you end up pretty much with Vanilla Coke.
As for those 7 gallons: If you want to experiment you can scale down the formula/syrup quite a bit to avoid that problem. Just add a drop or two of formula to a certain amount of water, and then add the same weight of sugar (eg., 100ml water = 100g sugar), and then continuing adding another drop of formula until the syrup balance is right. The citric acid and caramel colour isn't needed for taste experimentation. Now you can either mix some syrup with carbonated water to taste the mix, or you can just taste the syrup directly. (I bet a real flavourist with a sensitive palate will be able to just mix the formula with some still water and taste that without any sugar, though.)
The real problem is that you have to properly emulsify the formula when changing the balance of ingredients. But you can make the process easier by doing a base emulsion and using that to branch off of. For example, if you want to see how it tastes with more orange and less neroli, make a base formula with everything in place except no neroli, then pour off some of the formula, add a bit of orange, re-emulsify, and then make a small syrup batch from that. Time-consuming, sure, but doable.