the government should be allowed prevent people from spending money to buy ads expressing their opinion because in that case the regulation is on the person's expenditure of money, and money is not speech (or, if you prefer, money is not-speech).

Buckley v. Valeo disagrees.

The first amendment gives Americans the right to free speech.

More conveniently, it prohibits Congress from regulating the freedom of the press, i.e. the printing press, i.e. the technological means of reproducing ideas so that others may consume them, as in television ads.

Which is why I found the Citizens United decision so baffling- the reasoning they used to reach their conclusion was not at all the reasoning I would have used. (But, then again, I would rule the vast majority of laws Congress outputs unconstitutional, which is one of the many reasons I have not been nominated to the Supreme Court.)