I’ve never spent more than $250 for a bike. I’ve ridden that one for 15 years and something over 12,000 miles. Halfway around the world.
But why is spending big bucks on a bike any more foolish than spending even more on a motorcycle, sports car, boat or other indulgence?
If you rode it half way around the world, how did you get back?
Because, as you correctly observe, you can buy anywhere from around 10 to 25 very nice bikes for $7,000. That's why it's foolish. I myself spent $450 on a bike I've now owned for ten years. Before that, I owned a bike a spent around $500 on and used for 25 years [so in 1980 dollars, it was pretty salty. We were DINKs then. I wouldn't spend that kind of money now. Even so the amortized cost was $20/year.]
It's his money. He can buy whatever he wants with it. I don't say it's immoral, but at some point [I'd guess around about $1500-$2000] you aren't really paying for a bike anymore... And again, if whatever the extra $5 Large is for is what he needs it's his money to spend.