EUREKA!! You've invented the successor to sliced bread!! This will make your fortune -- if only you can protect it. Now what? This is a bifurcated, or two-part, step. Step 1A is to SHUT UP ABOUT IT. That's right, do NOT tell the world about your invention. Patent law in the USA provides for a one-year window after the first public disclosure of the invention. You don't want to put that clock on yourself or your patent attorney. Step 1B is to write your invention down. A one-page summary is often enough, but spell out the advantages of your invention, as well as how to make it and use it. Patentable subject matter includes UTILITY (the better mousetrap); DESIGN (non-functional design on a utilitarian device); and PLANT (on a plant, not a tuber, that can be asexually reproduced -- no patents on potatoes; sorry, Idaho). The cost of keeping your mouth shut? Free. No charge. The cost of writing your invention down? Your time.
Step 2 is generally to search the available literature for inventions that may be similar to yours. This step, in most cases, is voluntary, but it is highly recommended for two reasons. First, it provides some degree of assurance that your invention will not receive a rejection based on the prior existence of the invention. Second, if there is an invention that is similar to, but not identical to, yours out there, it gives you the knowledge you need to "invent around" that invention so that you do not infringe on that inventor's rights. This step is required if you are filing an application under the USPTO's accelerated examination program. Cost of a patent search: $1000 - $5000, but it's the best money you'll spend in the patent process, especially if it comes back with a result that would clearly make your invention unpatentable.
If your search shows that your invention is novel, useful and nonobvious, really consider the risks vs. the benefits of going through with the patent process, because this gets pricey from here on. While there are many do-it-yourself books, and you certainly have the right to proceed without a patent attorney, your application stands a much better chance of doing it right if you have a professional draft your application for you. Cost: Attorneys' fees can start at $2500 and go all the way up to $100,000+; these generally depend on the complexity of the invention. Filing fees start at $500 for a small entity and go up based on the number of claims.
Start with a LONG wait. The US Patent & Trademark Office has 1,000,000+ patent applications waiting to be examined. Your application may be shelved for several years before it is examined and a First Office Action is issued. An Office Action ("OA") may run the gamut from "OK, your application is approved" to "Your invention is unpatentable because ...". If the OA is based on something like your public disclosure of the invention more than one year prior to the filing date, you cannot get around that and your application is dead; no patent will issue (no fees are refundable). If the Office Action can be argued or overcome, your response should be to "traverse" the examiner's objection/rejection. Cost: Varies wildly, but generally several $thousands.
Well congratulations! Your application has been approved by the examiner and will issue as a patent -- after certain formalities are met. These include payment of all fees associated with the application, including an issuance fee. Once that fee is paid and the USPTO goes through a few more formal steps, the patent issues. You receive a beautifully bound copy and your friends may now throw you an issuance party. Cost: Issuance fees are around $900 for a small entity, $1800 for a large entity. Depending on the formalities involved, there may be attorneys' fees associated with issuance, but they will be minimal compared to the application and prosecution fees.
Issued patents cannot just sit there. The USPTO requires maintenance fees to be paid at 4, 7 and 11 years after issuance to keep the patent alive and in effect. Cost (for utility maintenance, which is the only patent type that requires maintenance fees): You can find these fees, which change quite regularly, at 37 C.F.R. 1.20(e)-(g). Currently (as of August, 2008) maintenance fees start at $465 for a small entity maintaining at 4 years and go up to $3,910 for a large entity maintaining at 11 years (a 7-year maintenance payment is also required).