i hate to say it... but i doubt trying to "shame" them by comparing the bike purchase to their car purchases is a good strategy. they earn their own money and have every right to spend it anyway they want...even foolishly. they will probably not look favorably on their kid reproaching them about money

you live by their rules when you accept their support. if your folks do not see the value of an expensive bike for a college kid they will not go for it. a trip to a good bike store with your folks to show the differences might be a better tact

many moons ago when i was in college i wanted to get a mountain bike and had saved up the money...there were the new thing and there was only one other guy on campus that had one. i paid my own living expenses as an undergrad but my father paid my expensive private college tuition. the mtb idea did not go over well to say the least. it frustrated me a "bit" as my father just bought a new caddy as well...but i respected him too much to argue about it

it took me a long time to figure it out...but my father was trying to teach my patience and the value of money. when i went to medical school (which i paid for myself)...the first thing i bought was a mtb...i really enjoyed it