This does depends on the level and skill of the developer that is writing the application, plus you must consider time to deliver the application and time to quality control.
On the cost you should also consider structure for servers and a designer. Most developers will do some design, but if you are looking for something up-scale you probably will need a designer to create a mock-up of the application.
You can ask the developer to sign a contract of confidentiality, therefore this will only force the developer to not reproduce the application in the exact content, which doesn't usually helps. You can ask a lawyer for more info in this point.
Anyway, in a good scenario you will be spending a developer year salary to get the first phase of the project out of the door and even if you use a framework as base, you will probably take about 3 months to get a first phase done (don't rush it, just makes it worst).
As advice I would suggest you to use a framework like Zend Framework since you will be using PHP & MySQL.