you've already believe that speech doesn't equal money, so I don't see why you would think money equals speech.The point is to make it affordable for everyone to have a roughly equal opportunity to monetarily engage in political discourse (if we entertain the rather absurd notion that money = speech). By removing any limits to contributions, Mark Cuban can buy off a politician who wouldn't even need to listen to my opposing viewpoint, because I simply don't have enough money to make it worth his time.
What you described is called lobbying, which isn't illegal. Unless you are describing where Cuban bribes the politician to do him favors, which is already illegal.
If you don't see how it's a problem to let only a wealthy few control the government, then I'm not sure you really belong in this discussion.
If all campaign contributions are public, then all the information is on the table. If everyone can only donate $50, you MUST put a monetary value on 'in-kind' donations, up to and including my voice, which is a ban on political speech. If you don't, then the wealthy can donate their $50 like everyone else to the campaign, while doing a "friends of X" campaign not in conjunction with the official campaign.
Do I think we need to reform campaign finance? Absolutely, by making all donations reportable and taxable.