There are moderate limits as to how much each person can give to individual campaigns. This prevents people from using their money to give undue influence on candidates. People can give much more money to more broader issues but these funds to not affect individual officials as much and as a result will not have influence on them.
You know there are several ways to bypass such limits - but if not, see a few examples here: http://www.usnews.com/opinion/economic-intelligence/2014/08/14/wall-streets-money-is-flooding-congress.
But out of curiosity, if you really, honestly believe candidates aren't under "undue influence" from big money interests, what's your explanation for, for example, the section dealing with the repeal of the Dodd-Frank derivatives rule (of which 70 of the 85 lines of text were, for all practical purposes, written by Citigroup). Now, I'm under the impression most people aren't happy about that section, so what happened - the people didn't pay them enough for them not to do it?
The vast majority of people can afford to give an "average" amount to a candidate they support and have their voice be heard
Such an "average" amount won't even begin to compare to how much they currently receive from wealthier donors - read the above link. Also, as the article I had linked in a previous post put it: reading that study, it's hard not to come to the conclusion that the US is pretty much an oligarchy. Again:
Quote
"The central point that emerges from our research is that economic elites and organised groups representing business interests have substantial independent impacts on US government policy, while mass-based interest groups and average citizens have little or no independent influence."
It's not like people aren't organizing and trying to make their voice heard - it's that they're simply not able to compete.