The culture in those days among scientists was not to make a lot of money or bother with the business details. They were scientists first. Alexander Flemming didn't patent penicillin. Jonas Salk didn't patent the polio virus. That's the way they did things back then. Guthrie and his hospital were naive about business, and they trusted Ames to do the right thing.
The NEJM article says that Guthrie produced 500 tests, at a cost of $6 each. That means the $6 covered the costs, including the house and everything else. He presumably hired workers to assemble the kits. It's hard to imagine a principal investigator assembling 500 test kits himself. When the U.S. Children's Bureau found out, they decided that the $262 was too high, and they revoked the deal.
There were congressional hearings, so if you want the details, you can look up the hearings, which might be online.
As Elisabeth Rosenthal said in her New York Times stories about the health industry, drug companies charge the highest prices the market will bear, not because they need the money for R&D or manufacturing costgs, but because they can. And that's what they say in their SEC filings and reports to their investors. You can buy asthma inhalers in the U.S. for $160 and the same inhalers in Europe for $5. This happens all the time. The FDA gave the rights to colchicine, a drug that has been used since the pyramids, to a private company, and they raised the price from 5 cents a pill to a dollar. You could still get it in Canada for 5 cents.
There are very competent people working in government labs, and if they want to, they can manufacture anything. Look at the Manhattan Project and the Apollo space program. The Centers for Disease Control provides testing services that aren't available commercially for rare infections.