I was wondering about that too. I think lobbying is only one side of it but there is also the threat of attacks ads from super PACs. They often don't even have to spend the money because the threat alone is enough. And that is much more money.

On issues where there are > 1 powerful organized constituencies in the lobbying market, this is exactly what happens.

However, many issues have just a single "market participant" which is organized enough to participate, and then the ROI is really high.

Market theory doesn't apply. The group against which lobbying efforts are directed is often the American people - and they are not organized enough to 'enter the market's.