A lot of the flavour pages on ELR cite the flavour's specific gravity. For example, if you go to:
You'll see this:
Manufacturer specified gravity: 1.038 g/ml
Although I'm not sure why you would need the specific gravities. Mixing assumes 1ml = 1g. If you enter the actual weight of the concentrate in your recipe calculator, you won't be getting the same juice as the recipe creator.