Okay.
To put that in perspective - the average Congressional district has around 750,000 people in it. 3% of that is 22,500 people. It costs about $2/signature to hire people to petition, so under your framework, there would be a $45,000 upfront cost for petitioning alone before a candidate would have access to the same resources as major candidates. With a $200 limit per donor, nobody who is not part of a major party will be able to raise that much money.
Honestly, this is an excellent question and I had not considered that people were hired for what I thought was volunteer work. But I can think of a few solutions.
Perhaps set the limit to $500 which most average people can still usually afford if they are passionate, or maybe allow $200 per quarter or month, or maybe have a more loose limit before the candidacy paperwork is turned in, I will have to think through this.
Let's set that aside for a minute - when these candidates qualify for public financing, how much are you going to give them? Remember that all of this money is coming out of our pockets.
This depends on the race of course. Presidential would be about $300 million. The average cost of a senate and house election in 2008 was $5.3 million and $1.1 million respectively. With the senate (and somewhat for the house) there would need to be some flexing depending on exact populations, but that seems to be a good amount.
Using 2008 as a standard, this (assuming 4 candidates for each race, 5 for presidency, which seems reasonable to me) works out to about $4.1 billion. I estimate about $2.6 billion in a nonpresidential year such as 2010. Not really that much given the scale of the US federal budget.
Pretend that this decision and the 35 years of jurisprudence explicitly defining money as speech never happened. How it would work in your ideal world?
Obviously money has a lot to do with speech. I cannot deny that. Nor can I deny the reasoning that the courts use, which is pretty obvious to me. I do think that the way the money is generate is what causes what I believe a perversion of our system away from the average person and more to wealthy causes (or the causes of the wealthy) causing unequal representation of the citizenry, thus causing have some to have less speech than others.
There should be nothing preventing a corporation, charity, etc from running ads saying stuff like "abortion is wrong" or "we dislike prayer in schools" as long as there is full disclosure of funding, and the message is not tied to another candidate, group, regulation, specific policy proposal or existing law. So they could not say person or group X is good or bad for Y reasons. Any facts used should be cited at the end of the commercial and accessible on the web or other inquiry. This, I believe, would direct advocacy towards citizens and not politicians but would cut out the inflammatory and inaccurate speech the both parties currently use, who would than vote for their priorities against a greater number of candidates. In the case of no majority, I don't know what would happen at the national level since I don't think it has ever happened, but at the local level, have a runoff of some type.
In terms of affect, I don't know what would happen, but I think it would make the government more responsive to its citizen's concerns. Whether that means things go more conservative, liberal, or some other direction, like libertarian, I can not say and really isn't important as long as the government does a better job of representing its citizens.