Op is definitely being unreasonable, because you did not get ripped off.

It takes an inexperienced builder about 10 hours to put together a typical ARF. I'm sure the guy at the shop did it faster, but he saved you the 10 hours. Then he set it all up, and took on the risk himself of test flying it for you. A bad connection or radio problem would have cost HIM an airplane, not you. Not to mention that he likely trimmed it out, set the control throws properly, tuned the engine, and got the CG right. All told, he probably saved you 15 hours or more plus the risk of a bad first flight. I wouldn't do all of that for $300. He charged you a reasonable fee for the work he did, and in return you got a plane that is a proven good flyer that doesn't need anything done to it besides minor tweaking to suit your taste. Do you really think the shop that would assemble your plane for $100 would give you that?