Anybody have a recommendation for an A/B testing service?

We've talked to Optimizely but their pricing was going to come in at the same ballpark as our AWS spend (into the six-figure range), which seems absurd. They charge based on monthly users, but a lot of our traffic consists of organic search bounces.

For now we just want to run ~5 experiments per month, want to record events server-side so we can be sure not to lose any, and are wary about implementing it ourselves since there are so many ways to screw it up without realizing it.