“How much work did he put into it to charge that extra amount? I seriously doubt that much, if any, maybe a few phone calls.”
That particular bag might not have been much work. If he knows a little about how sugar is made and packaged then he might have just made one phone call to determine that it has an OU. But, his entire line of Kosher products does take work. It takes knowledge of kashrus standards.
As I described above, the whole point (in my understanding) of heimish hachsherim are for those who don’t fully understand their own chumras. You want to be machmir (especially for Pesach) but you don’t know enough to know what the chumras are so you outsource the chumras to the Heimish Rav Hamachshir and he makes the decisions. He decides that OU sugar is good but OU widgets are not, he puts his sticker on the OU sugar and you pay extra for it. He did the work, he learned the halachos, he applied the halachos to decide what meets his standards, and he went around putting stickers displaying such, and you are paying him for it.
To some extent the entire kashrus industry is like this. In the old days people would buy a bag of sugar with NO hechsher at all. They understood how it was processed, packaged, etc and therefore understood that it was kosher without even an OU.
Nowadays, with all the random ingredients and machines that go into processed foods, even many knowledgeable people don’t know how to tell if something is Kosher. That is why we pay big money for companies like the OU to pasken for us.
Side note: There are some who still pasken based on ingredients as pointed out at the end of your post:
“There is a infamous web site that is matir almost anything by checking the ingredients.”
There was a thread about this: