0.5 mL and 1 mL describe how much filler is in the syringe, not a finished lip shape. One millilitre is twice the volume of half a millilitre. That does not mean twice the visible change.
The FDA says results depend on the tissue you start with and on the amount and type of filler. Cleveland Clinic describes about 1 mL as an average, which is not a starting dose for every person. Placement matters too. The same volume at the edge, the Cupid's bow or the corners is not the same plan.
Don't assume half the volume means half the fee either. Ask what the quoted price includes, and whether a later review would come before anyone talks about more.
LookPrep has a page on what 0.5 mL versus 1 mL of lip filler actually changes, with illustrated amounts on a demo face. Planning pictures, not a recommended dose.
Sofia R., LookPrep