It's easy to look at a one-off deployment of a single server and remark on how much cheaper it is than RDS, and that's fine if that's all you need. But it completely skips past the reality of a real life resilient database server deployment: handling upgrades, disk failures, backups, hot standbys, encryption key management, keeping deployment scripts up to date, hardware support contracts and vendor management, the disaster recovery testing for the multi-site SAN fabric with fibre channel switches and redundant dedicated fibre, etc. Before the cloud, we actually had a staff member who was entirely dedicated to managing the database servers.
Plus as a bonus, not ever having to get up at 2AM and drive down to a data centre because there was a power failure due to a generator not kicking in, and it turns out the data centre hadn't adequately planned for the amount of remote hands techs they'd need in that scenario...
RDS is expensive on paper, but to get the same level of guarantees either yourself or through another provider always seems to end up costing about the same as RDS.