If there's a broader argument I would make against the argument I think you're making its that I don't think history has held up what seems to be the fundamental assumption: that markets do not naturally optimize towards something close to a monopolistic system.
I had to butcher DF's point a bit, given it is several hundred words long originally. You can find it in full on page 56 here (
http://www.daviddfriedman.com/The_Machinery_of_Freedom_.pdf). When I said X% of the economy, I meant whatever was for sale. We could be talking about buying cars or apples or something. It would be far more proportionate to determine their ownership by dollars than by votes. Especially when you consider the economy as a whole where there are innumerable goods and services.
As for your second point, you're missing two assumptions. Firstly, you're assuming that the House has, say, 75% Republicans and 25% Democrats, but I used the word elections. If you have that 75/25 split in each constituency that would give you a 100% Republican House. You could say that there would still be debate even then as not all Republicans agree anyway, but this is not a universal rule.
Speaking about the US you are talking about what is called a weak party system, where there isn't much hierarchy and it's relatively difficult for a politician to get kicked out for ideological reasons. If you were talking about countries that have strong party systems, then they have official positions which it is very dangerous to go against. In the UK, a party can get win a majority of seats with 35% of the vote, and it is fairly uncommon for MPs to rebel against their party leader (who would also be Prime Minister, we don't have much in the way of separation of powers). Therefore 35% get 100% of the votes in the House of Commons.
Imagine if Coke had 35% of the cola market and Pepsi 65%, and yet Coke ended up with 100% of the market. It's a paradox in the private sphere but not in the political sphere. Tweak the percentages there a bit and it would be true in an American comparison too.
Could you elaborate what you mean by (presumably free) markets tending towards monopoly, and the ways in which businesses pass on 'the costs' as a way of increasing profits?