I played a game at lunch today where a level 2 guy bought out a level 5 competitor simply due to a brief period of high cash flow. A couple minutes later, he upgraded to level 5 and steamrolled the remainders. This sort of situation makes no sense at all. I think the biggest issue may simply be that stock value does not respond quickly enough to buying pressure. If stock prices went up faster due to buying pressure, these sorts of very quick buyouts would not happen. It would put less game value on quickly building up cash to buy stocks and more onto making an economic machine that can fuel a long buying campaign. There is no real legitimate reason a level 2 company should be able to buy out a level 5 company when they cannot even get themselves upgraded to level 5 for the same cost as buying someone who is already there. Each level upgrade should have more of an impact on stock price and each stock purchase should have more of an impact. And, cash on hand should be weighted more fairly, as well.
Did you consider, that this level 2 guy simply has better understanding of the game, cos you need a significant cash flow to buy out a level 5 guy unless he has so much debt that he costs nothing... that means he had to be producing the most efficient things from cash point of view, meaning he went towards a much riskier strategy which simply paid off... You should instead try understand his strategy, and I assure, good players choose when they upgrade, and when doing something else is better, just shows the game has more dimensions.
Also the thing about cash on hand, is like Soren said, if you added cash on hand to the stock price, then the game can just last forever if both players have a very even income rate. In the current system you simply have to race for victory, which pretty much guarantees a short game.
And yeah, the whole buyouting is not realistic, but I'm fine, because I see it as a game, which has player use some of their economic skill to outplay each other.
about resource values, I think they are valued of how much you would get if you would just outright sell them, if you a lot of some resource, then after the first couple hundred, all the rest cost 1$ regardless, so if you sell things gradually for good price then you will get the best value out of resources.