Here is another solution that hasn't been mentioned yet, but has by far the best price/performance if it matches your use-case. Google BigQuery isn't advertised as being for log search, but in practice it works phenomenally well. It provides exceptionally low storage costs, combined with a powerful query language and reasonable query costs. The counter-intuitive part is that the query performance, even on tens or hundreds of gigabytes of data is amazing, and better in practice than many purpose built inverted index log search systems.

If you want to use your logs for troubleshooting (e.g. ad-hoc queries to find error messages) or ad-hoc analytics it is ideal. Hundreds of gigabytes can be searched or analyzed in 5-6 seconds per query.

Fluentd can be used to collect log data and send to BigQuery.