It's 300 health, 120 mana for 225 gold. After your first back it probably isn't necessary, but even if it is it's 600 health, and 240 mana.
Before your first back that comes out to be: 70g worth of health pots, and ~35g in mana potions. totaling in ~110g worth of healing. Including your first back: 220g worth. If you didn't back, you could have just bought 4 health pots, and 2 mana pots giving you 600 hp, and 200 mana at a cheaper rate.
Sustain isn't bad, but I don't think the flask is ever worth it, personally.