It's odd that they don't explain it on their rate chart, but then again, their pricing approach is confusing to newcomers. They sell inbound calling separate from outbound calling. E911 service is offered with their outbound calling plans. E911 is included at no extra charge on all of their plans with buckets of minutes, so there's no fee listed. Personally, I use the North America 500 plan, but even their 120 minute Basic plan for $1.95/month comes with E911 included. Perhaps that plan would be ideal for you? I think of it as E911 for $1.50 + 120 min. for 45 cents.
It used to be that the pay-per call plan would have an optional $1.50/month fee for E911, but I don't see it spelled out. If you start the signup process for the PPM option, and it doesn't have the $1.50 E911 option, you can always email them and ask.