Injection molds are generally water cooled with a 30-90 second cycle time, depending on the amount of plastic to be cooled. Depending on the injection machine, they can range from 12"X18" to 24"X36" and be in the vacinity of 4" thick.
Check out Garst Lure Design. He designs molds for some of the biggest in the industry. Design costs and final cut of a mold can run upwards of $15,000 or more depending what you want and just how much you want to spend.
Something else to consider is when injecting salt, you are wearing away the mold, much the same as using an abbrasive.
I have a few contact that cut molds for customers.