The cost of a provisional patent application will depend on complexity, technology, completeness, and strategy:
Complex inventions with many parts require a lot of time to describe each element as well as how the elements work together. Conversely, simple inventions with fewer parts will be cheaper. The more complex the invention, the greater the cost.
The technology of your invention will play a large role. Simple mechanical inventions will typically be cheaper. Cutting-edge biotech or telecommunication networking systems may require more attorney time to not only get up to speed and understand the details of the invention, but also to articulate those details in a draft.
A provisional is only as good as its contents. In most cases, a robust and complete disclosure (“A”) having a detailed description, a professional set of drawings and claims is ideal. A “thin” disclosure (“B”) having holes in the description, few drawings, and no claims may not offer the protection you desire. You should not expect to pay the same amount for application “A”, which is complete, and application “B” which is thin. While budget may dictate that “B” is the most that you can afford, understand that it will come with risks, and that any deficiencies will have to addressed later on in the process.
Some clients simply want to be able to market their product as “Patent Pending.” Patent Pending means that an application has been filed, but it does not at all give any indication whether a patent will be eventually granted. If the sole purpose of the provisional application is marketing, then the cost will be low. If, however, you intend to eventually license the invention to others, or enforce a granted patent against competitors, then more attention to detail is required.