Unfortunately 715 FG is not worth $20. That may be the rate that you can get it from donating, but the usual ratio is about 1$:100 fg.
This rate is due to the going rate of Set cost items vs fg (CS:GO/TF/ect. Case keys are $2.50 and trade for around 250 FG, same with Magic The Gathering Online Tickets $1.00 vs 100 fg)