You're probably going to want a patent attorney to handle your business.
If you're filing for a provisional patent, you're looking at 3-5K. A provisional application will secure a filing date and then you'll have a year to convert your provisional application into a full-fledged application. If you're filing for a full-fledged application from the onset, it could be around 10K or more, depending on how much work is required by the lawyer. A good idea would be for one lawyer to draft the application and then spend 1-2K for another lawyer to give you his opinion before you actually file.
In a nutshell, the cost is a function of the work to be done and therefore the more upfront work you put in yourself, the less time an attorney will take. The figures I gave are based on my own experience and I'm sure the actual cost for your patent will be different.