There are 4 primary methods, and you need to understand the difference, as many companies' methods are not transparent and often very misleading.
Flat Fees - All-Inclusive: A flat fee that includes all required services to complete your case is the most reliable way to immediately know the full cost of your case before committing to work with any tax relief company. It also assures that you are working with a company that has professionals on staff that can handle all services associated with your case.
Flat fees - Non-Inclusive: Many firms will quote a low fee, but downplay or outright neglect to tell you that they don't provide some critical services. Ask if your "flat" fee covers all of the services required to resolve you tax issue. Bookkeeping, audit representation and filing your back tax returns are common services that some tax relief companies exclude. Failure to include these services can cause delays and unexpected costs to your case, not to mention the inconvenience of having to find additional professionals to get the work done.
Teaser Fees: Some tax relief firms take advantage of price shoppers by selling them on what appears to be a reasonable upfront "investigation fee," but then follow up days or weeks into the process to inform them that the remainder of the case is going to be additional $$$$.
Ongoing Fees: Other companies just bill you by the hour, which is by nature a conflict of interest leading to unpredictable, ever growing costs and slow progress of your case.