NOTE: (I'm not suggesting this because I'm "cheap". I own several paid heroes.)
Whenever this is suggested, people ramble on about how ironhide can't make any money if heroes are essentially free. There's an easy solution to this. Make the hero-gem prices extremely high. Simply make the exchange rate 20000 gems for the equivalent of a dollar. That's 120k for a hero like Phoenix, and only very dedicated players will ever be able to get that.