Solo kills need to be worth 450, like kills + assists are.
This is one of the reasons for the huge disparity between a snowballing top laner and a snowballing bot and mid laner. Say top totally stomps and goes 4-0 in lane. If a bot lane wins by the same amount, the adc will have 8 KP, if not 8 kills, and each of those kills will be worth 450 gold for the team instead of 300. In addition, the team with the winning bot lane has complete control of early dragons, which are key to winning the game.
IMO, this is one of the biggest problems with the influence disparity between Top and Bot Lane.