It takes a lot of money to form and to run a successful interest group because all of the things that interest groups do require money. In order to see why this is so, let us first examine the things that interest groups do.
The main goal of interest groups is to influence government officials. Interest groups want to make their opinions known to people in government and they want the people in government to pursue policies that are in line with what the interest groups want. This entails a lot of things that require money.
Interest groups need to have strong organizations in order to be effective. They have to have offices with various workers who are in charge of things like getting new members, raising money, and generally running the whole enterprise. This takes money.
Interest groups need to have facts to bring to government officials and lobbyists to present those facts. This can mean that interest groups need to pay for studies that will show that their preferred policy positions are the best options. It also means that they need to have lobbyists who can get access to important government officials and can be persuasive when they get that access. Such lobbyists do not come cheap.
Interest groups also need money for various things related to their lobbying. They need to have money to spend on advertising that will support or oppose specific candidates or issues. They need to have money to donate to candidates and to other political groups so that they can cultivate the good will of the people to whom they donate.
If interest groups do not do all of these things, they will not be successful. All of these things require money and therefore interest groups cannot generally succeed unless they have a lot of money.
Politicans may not cost much to buy, but the real challenge is keeping them bought. That's why the checks have to keep going out - sorta like paying your cable bill every month - it adds up after awhile, especially if you're buying a dozen or two politicians.