I doubt that a roaster who sells the odd kilo of green beans @ around $10 is really trying to make a killing on it. The ones selling at $20/kilo and over for basic beans ARE trying to make a killing.
Your roaster sounds more likely to be passionate about coffee and keen to share the experience.
Yes, ultimately it is a business, but most roasters would be buying their green beans in bulk which is likely to have them (at present) a fair margin under the $10/kilo mark.
I think it comes down to the type of person youre dealing with and the kind of relationship you can foster with them. I think common sense plays a big part too. EG: If you are spending time talking to him that is keeping him away from other aspects of his business that ARE his bread and butter, he might not want to continue, but if you are coming in adn having a friendly chat when he is not too busy and at the end of it you buy 1 or 2 kilos of beans from him, then no harm/no foul.
If neither of you are trying to rip the other one off, and you are both happy with the outcome, then why wouldnt the roaster continue to sell you the odd kilo of beans?