what is a fair % for Tax lawyer

[Zulf H.]

Hi There, I would like to know what is a fair and typical  % I should pay the property tax lawyers for appealing my properties. They take a certain amount if they are successful in lowering the property taxes but I feel its high and I have given them 15+ bldgs to assess and submit. 

Wanted to know what other owners are paying for this service. I don't do it myself cause it seems time consuming and maybe a bit complicated. 

Thanks for the advice. 

ZH.