I think one of the reasons politicians can be bought so cheaply by interest groups is that the opponents of the interests groups have practically no money. The interest groups don't need to spend a ton as long as they spend more than their opponents.

The linked post talks about the effectiveness of AIPAC but fails to mention how much is spent by say, Palestinian interest groups. Perhaps there's a good reason for this: do Palestinian groups have any money to spend on US elections? Try fundraising in Gaza right now.

Likewise, business interest groups have a lot more money to spend on elections than, say, environmental groups. The latter have to beg for small donations from individuals just to stay afloat. Thus, it's relatively easy for business groups to outspend environmental groups. To win an auction, you just have to be the highest bidder.