To me, they're not really the same thing. MySQL is more of a replacement for xBase than a proper enterprise product, it's certainly nowhere like Oracle or SQL Server in terms of support for critical systems where your data "matters". Obviously though, the price points are also worlds apart.

MySQL is simpler and regarded as faster due to less overhead for smaller projects. For a Mom n' Pop's corner shop online presence where if the site dies you can rebuild it without breaking into a sweat and no great demands on it, MySQL fits the bill. You wouldn't want them to be storing your medical records in it though (at least I would be horrified at the idea). Certainly a paid license version of Oracle would be colossal overkill and as mentioned above would add both cost and complexity to a small project.

If your data is "important", frequently updated with minimal dataloss requirements and transactional and referential consistency, that's where Oracle would come in.