Downside of servermojo is if you have 800 domains they won't check them very often, if I'm reading this page correctly:
https://www.servermojo.com/cgi-bin/signup.pl
Big Biz - 2500/hour - $200/month
Is that 2500/hour, per monitor/URL? (A check every 1.44 seconds)
Or 2500 spread across your 800 domains?
Based on that they would get checked every 20 minutes or so. You'll have to do the math.
For uptime monitoring/stats, I'd rather be monitored every minute.