I think there are 2 separate ideas being conflated here. One is the right to spend your money however you want to, which I agree is fine. The other is whether whether spending money is a first amendment right to free speech. I think this stretches the meaning of free speech a bit too far.
It seems almost impossible to separate these issues, though. If the Koch brothers decide to make a documentary about Obama, but are limited on how much they can spend making the documentary, isn't that limitation on free speech?
I am highly critical of the corrupting influence that money has on politics. I don't want to see this country become an oligarchy.
However, there will always be economic inequality. I don't think it's a question of whether the rich have more free speech than the rest of us. They do, however, have a greater ability to promote their views. They also have a greater ability to buy big houses and send their kids to better schools. It's one of the benefits of being rich. And yeah, it bothers me.
But at the same time, the guy from Occupy who camps out in front of LA City Hall also has a greater ability to promote his views than I do, because I have to go to work and can't afford play guitar in front of my tent all day.
I'm not really sure what the answer is, but it seems that by limiting the amount that people can spend on speech you will affect the content.