Patent law

Another difference between a provisional patent application and a regular utility patent application is that the drawings don't have to be in a formal, camera-ready printable form. However, they nevertheless have to be legible when scanned. Beware that although a provisional patent application need not have formal claims, it will not be of much value unless it meets the judge-made requirement of including a "written description" of whatever invention(s) are going to be claimed when the regular, nonprovisional application is eventually filed.

At current rates, for an individual inventor or business having fewer than 500 employees, the government fee for a provisional patent application having 100 pages or fewer is $110.00 whereas the government fee for a regular, nonprovisional utility patent application would be from $565.00 on up, depending on the number of claims and the number of pages of text. See link.

The cheapest way to file a patent application would be to write it yourself. I don't recommend that strategy any more than I would doing brain surgery on yourself, but for those who are willing to do the work and take the risks, a good start would be to read the latest version of David Pressman's book Patent It Yourself. See link.

You didn't mention what field your invention is in nor how complex it is. I'd consider the rule-of-thumb figures that Raj posted in the previous answer to apply to the entire cost of preparing and prosecuting a typical patent application. In my view (from the perspective of a patent attorney near Philadelphia), the initial cost of preparing and filing a patent application for a relatively simple invention might be about half that. But beware that if your disclosure as initially filed is later found by a court not to meet the stringent requirements of the U.S. patent laws, then whatever you have invested up to then would turn out to be valueless. For example, see the case of New Railhead Mfg. Co. v. Vermeer Mfg. Co., links attached. The links are to the case decision as issued by the court and to an article about it in the journal which I edit, Biotechnology Law Report.