Okay, so maybe it should be structured differently. Like a voucher system. Everyone gets, say 5 vouchers per election cycle they can donate to any candidate or party they would like. With those vouchers, candidates or parties can "buy" airtime/billboards/whatever. Let's not tie this to major parties only, please. I personally don't give a shit if the American Nazi Party gets airtime. We have to rely on people to filter out the bullshit on their own, otherwise we'll necessarily end up with censorship.
The main take-away is that money is not an equalizer and should not be relied upon for a functioning democracy.