A quick search of online sites, Leslies has PhosFree 2 L for $32, 2 L Pool Perfect for $42. Mailorder sources were $18 and $23 respectively.
Another search of dosage says " "20,000 gal pool with 300 ppb phosphates need initial dose of 1 Liter."
dosage of Pool Perfect for same pool would be 0.1 Liter per week.
So, for that pool as described, $16 initially, plus $4.20 per week, per Leslie's pricing
$234/yr if you add the stuff all year.