The OP did not say which type of car it was, but did mention it had a fob in addition to the key. So it could be anything price wise depending on what the $25 a day car had. Some remotes are like $20 or $30 and can be programmed doing the "pedal dance" without a visit to the dealer, though probably not in this case as there is no other key to use to do the programming. On the other end the remotes are hundreds of dollars and require extensive programming that takes the car out of service while the computer is reprogrammed.

There is however very little chance Hertz is passing along any discounts they get in buying a new key, non insured things like this are a major money maker for them.