How much should you sell 20 mg adderall XR for?

I'm in college and a 30mg ir can go for around $4-5, and xr's I can find 20mg for $3, but normally go for $4 as well. Vyvanse normally runs a dollar per 10mg's. Adderall differs based on supply and demand. For example during finals, because of the large demand all prices normally rise by $1-2!