First of all, the PTO will expect you to pay a filing fee on your patent application. Fortunately though, the filing fee is not due immediately. You may choose to send in the filing fees later during the prosecution. There are different fees for different application types. Different fees for various documents. And different fees for the petitions that may need to be sent in during the prosecution.

As you can imagine, the mere filing of a patent application is a fairly expensive ordeal. To make matters worse, the PTO actually charges fees throughout the entire process. At the very least, you should expect to pay a filing fee and an issue fee (but remember, you won't need to worry about this right away - it can take well over 2 years for your patent to be allowed).

Maintenance fees must also be paid on every utility and plant patent at three intervals once it is granted. At worst, you may have to pay these fees (filing fees, issue fees and maintenance fees) plus fees for several petitions, fees for late filed IDS's, extensions of time, disclaimers, appeal-related fees and fees for a request for continuing examination among many other fee related extras. It is best to know the fees associated with gaining a patent up front so that you can budget your expenses later.