I assume you' ve looked at FortiCloud? That might be the least expensive solution if it works for you.
If you have some facility with setting up a syslog server, I' ve found that Logstash can do a nice job of parsing the existing Fortigate logs.
This is really a DIY approach to interpret logs, though, and not an out-of-the-box solution. It doesn' t generate reports as much as it allows you to create specific views into firewall activity. Still -- it has all the data from your logs.
I' ve heard good things about the free Cyberoam Iview software, but have not used it. That' s probably similar to ManageEngine.
Logmojo.com looks like a good non-free solution and is tailored for Fortigate.
Hope that helps.
I' m a big fan of Logstash, but it' s not for everyone.
Bill
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Fortigate 600C 5.0.12, 111C 5.0.2
Logstash 1.4.1