How is the value of a corporation determined? (Golden Rainbow)

I've got a publicly owned electric corporation, it's one of the top producers of positive income every month at 1.82B (Yeah, I know, it's not a lot for you veterans), but it's got the lowest value at 1.01B.

Meanwhile my Building Materials has a value of 443.02B but only a positive income of 1.11B

So what makes the building material corp have a value that is so much higher than the electric corp?