It depends on how much you want to invest in this, however, companies pay top dollars for services like these (see: http://www.keynote.com/ )
Ehhhh, not so much,
I run IT ops at a startup, and $200-400/month was too much for PagerDuty or Pingdom's Team pricing levels.
We settled on their $50/month plan. Sure, enterprise customers may pay more, granted, but this is a commodity space. Ping, up? Good. Ping, down? Email, sms, webook. Race to the bottom.
Disclaimer: Technology professional, 12 years in the sysadmin/devops/mgmt "space".