I used agave syrup in an American wheat beer as a fermentable and then used it for priming. I wanted to get the Tequila essence.
The SG of Agave syrup is around 1.300. Other usual priming sugars are: Munton's Light DME is 1.043; Corn sugar is 1.038; Table sugar is 1.046; Belgian clear Candi sugar is 1.046. Honey is between 1.350-1.400.
When I figured out how much agave I needed (not being a math whiz) I looked at the % difference between agave syrup and corn sugar (1.300 SG ÷ 1.038 SG = 1.25). I measure my prime sugar by mass, not by the volume (¾ cup/5 gal) to get the Vol of CO² I want. I would have used 113 g of corn sugar for to get around 2.2 vol CO². I divided that by 1.25 and got 90.4 g of agave syrup. I found the CO² level to be great for a wheat beer and the taste is great.
The beer had a medium mouthfeel with a medium finish, not to dry, not to sweet, but didn't have any Tequila essence.