That's not really the argument.

Money's not speech, nor is a car or a bullhorn. But restrictions on these things can restrict speech. For instance, a uniform sales tax on items that include bullhorns is not, in my opinion, prior restraint. But requiring a special permit or tax stamp for someone to have a bullhorn at a protest obviously is. I think that's pretty uncontroversial.

Now, s/bullhorn/money. Same argument, but for some reason it is much more controversial.