For residential property tax appeal consulting the cost falls in a narrow range. Your best bet is to find the most qualified consultant that you can. Typical residential flat fees may be $350 or higher. Alternatively, many companies charge $250 up front and 25% of whatever tax savings are generated. Some offer $150 +/- plus a third of the tax savings generated. Generally speaking qualifications are more important than saving a few dollars in up-front cost.
Beware of companies that send you a letter or postcard that says "this is how much we think we can save you $." You can be reasonably certain they are using a database of old values to generate these postcard savings estimates. I would be very cautious about this, especially if there’s a high flat fee involved. Some of these companies may charge $350 or more, and if you’re in a city or county that reappraises on an annual basis, spending that much money may be a losing proposition. If they get a reduction that doesn't cover their service fees, and the County increases your tax assessment the next year, then you have just wasted your money.