The type of invention and your readiness for a patent significantly determine the cost of a patent application.
First - The type of invention. Three types of patents exist.
1. A utility patent protects a working type of invention, i.e., something that has a non-ornamental purpose. Utility patent applications require four different presentations of the invention, a short abstract, a written description of about 15 to 55 pages, drawings usually of 1 to 20 pages), and the claims- the part that uses really weird language, and is usually 2 to 15 pages. Added together, a utility patent application typically has at least 20 pages, and more typically, 40 to 80 pages. On average, each page requires an hour to 90 minutes in preparation. The hourly rate of the attorney or law firm thus significantly affects the cost, so the costs can run from $4000 to $8000 for a solo attorney to well over $15,000 for a lawyer at a large law firm. . A solo attorney will likely be less costly because solo attorneys are not sharing their fees with a partner or firm owner.
2. A design patent protects the appearance of an invention. The patent application has two parts - drawings, and one page ("specification") with a title, a listing of the drawings, and a short description. A design patent costs the costs of the drawings, plus fees for the specification page, document preparation, and filing. A complex invention could require many drawings, which, because the rules are complex and strict, should be made only by a good patent illustrator, and will cost from $100 to $150 per page, for a total cost of $1500 or more. DO NOT hire an off-shore illustrator for a design patent. Poor quality drawings can be fatal to a design patent. .
3. A plant patent protects qualified plants. It's a small and complex market so most patent lawyers do not handle plant patents.
Second - Your readiness for a patent.
If the invention fits a utility patent, a patent attorney can often work with you to prepare and file a provisional patent application. This is important because the provisional patent application should contain every detail know or suspected about the invention because the provisional patent application sets the stage for filing a non-provisional patent application. A provisional patent application gets its name because provisional means temporary, and a provisional patent application has a one-yeat life - just enough time to do more development work, marketing, and maybe find funding partners, with enough time (about 3 months) to prepare a non-provisional patent application to secure a patent. A provisional patent application costs from about $350 to $500. The online companies may cost less but you have a higher risk of not getting a patent because of an incomplete provisional patent application.