Surveymonkey did $45m in 2009 revenue with 20m monthly uniques which is $2.25 yearly for 1 monthly visitor. Wufoo does about 1.5m in monthly uniques which is about $3.5m/yearly revenue.
On revenue comps, if we assume 5x-10x multiples, the purchase price implies a $3 to $7m/yr.
Based on the $30 plan being most popular, and 10% of users in the most expensive plan, I'm assuming a blended $52/mo or $624/yr per subscriber. With our $3m to $7m revenue range, that implies 5k to 11k paid users. With a 5% conversion from free to paid, that is about 100k to 200k free users. Which implies a 6%-12% visitor to registered user conversion based on their 1.5m visitors/mo. Seems like in the range!
Finally, for returns, PG and Buchheit invested 100k for 10% (mixergy interview), and YC took 7%. So assuming a 20% option pool, and net of the angels, the 3 founder got $22m or about $1.5m/year.