"Being normal" is not a justification in and of itself, and is not an argument. There has to be a good reasons/reason for that normality.
He already tried to explain, with example, why the normality of decreasing price in games is due to those games being no longer developed or updated, and the devs have recouped their costs and just want to squeeze out remaining profit. He also gave a counter example which named a well known game which hasn't decreased in price (and month to month cost has increased) because of it's ongoing development .
He doesn't have to justify anything. It isn't his game. He didn't increase the price. He isn't the one complaining. You're the one who thinks something is wrong, and you haven't cited any actual reason why its wrong except that you don't personally like it and "it isn't normal". He was only asking that you provide something more substantial to support and explain your position.
(And granted, he gave up being polite about it)