But the cost of campaigns downs basically linearly with number of voters too. (TV ads are proportional to eyeballs.) There are some non-linear effects here, but I'm not sure how they operate.
You could probably get the same result by blocking funding external to the district and killing citizens united. And/or shorten campaigns to 60 days so they just don't cost as much. And/or, provide public funding and election spending caps.
I don't think that trying to scale out of it would be that effective but I see what they are getting at. My guess, is that would raise the inequality of the playing field again as the richest would be able to spin up an organization to manage 10k reps, decide whose relevant who to fund, and so on, people who didn't have lobbyists before won't be able to afford that even if the per rep cost gets cheaper, the total influence cost will rise and price additional people out of the market.
What makes you say that? Hell at 10,000 constituents you borderline don't even need to run ads, just go door to door, personally.
EDIT: Although I suppose the tactic could at that point be turned to buying voters more directly. Spend a ton of money on something that benefits everyone in the district and make it clear that it'll keep coming only if they vote for your candidate, and you might have the same effect.