It's great Apple pretty much delivers identical devices on a same product line except on the disk space. It's more than the disks for laptops but this means everyone pretty much gets the same experience even if you get the cheapest version and you don't have to go through the technical spec sheet that most people don't understand to figure what you're missing from the more expensive versions or you're physically missing on some features.
You'd rather get the same stuff at cheaper price if you don't care about disk space. It's not the disk space is more expensive than market price...
He's not wrong. The RAM is definitely more expensive to buy from Apple, to the point that I buy the iMac with the base 8GB and then replace with aftermarket RAM and save hundreds of dollars (approximately 15-20% of the price of the entire system).
Fortunately, servicing the iMac RAM is easy in present generations.