In FR/LG: go to Fuchsia City and then double-click the warp at the top of the map that leads to the Safari Zone gatehouse (11.10). Inside, there will be three green squares labeled 'S'. Decompile the script on one of them (they should all be the same). Search the script for 0x1F4 (500 in decimal) and replace any instances of 0x1F4 with the hexadecimal equivalent with your new price. It should show up twice; once in a 'checkmoney' command (which checks to see if the player even has enough money to enter) and a 'paymoney' command (which actually takes the money away from the player).
In Emerald: go to Route 121 and use the warp there to enter the Safari Zone gatehouse (23.0). There should be one green square labeled 'S' inside which you should decompile. Search the document for 0x1F4 and replace any instance of it with your new value (remember to convert to hex!).
Also, keep in mind that you'll want to change the text too because it mentions $500 in there somewhere.