Akismet

Monthly
Yearly (one month free)

up to 5 blogs: $50 a month
up to 10 blogs: $80 a month
up to 25 blogs: $150 a month
up to 50 blogs: $200 a month
up to 100 blogs: $250 a month
up to 250 blogs: $375 a month
up to 500 blogs: $500 a month
up to 1000 blogs: $750 a month

up to 5 blogs: $550 a year
up to 10 blogs: $880 a year
up to 25 blogs: $1650 a year
up to 50 blogs: $2200 a year
up to 100 blogs: $2750 a year
up to 250 blogs: $4125 a year
up to 500 blogs: $5500 a year
up to 1000 blogs: $8250 a year