Why is Vincent Vega so concerned about a $5 milkshake, when he had just bought $300 worth of heroin from Lance?

He's as normal as they come. Everybody has something (a hobby, a habit, what have you) that they value highly. Sometimes that thing they do winds up costing them a fortune and everybody around them - friends, family, or anybody - starts cocking eyebrows. "You spent how much on what!?" But the hobby of one person seems foolish to another.

To Vince, heroin is worth $300; a milkshake is not worth $5. He can't understand why somebody else would drop that kind of change for "just" a milkshake. Mia likes it (and does not care about spending money at ALL), so it's worth the $5.

Vincent doesn't have a lot of self-reflection. This is evident from his conversations with Jules about the deeper meaning of it all. It wouldn't occur to him to think about spending habits like that. Just like it doesn't occur to Mia to care about the costs and consequences of things; she'll drop $5 of (presumably) Marsellus' money on a 'shake just as easily as she'll use any drugs she finds in Vince's coat without asking or checking with him what the substance even is.

he thought it was overpriced whereas the heroin he bought was as top-shelf as they come.

after he tried the milkshake, however, he agreed with Mia that it was "damn good"
also, i wouldn't put too much deep meaning into the dinner scene since both characters were smacked out of their minds on drugs......

heh.

I wonder what Vincent thinks of me spending $5 on each Real Ghostbuster action figure I owned as a kid?

Do you expect reason from someone in constant inebriation with heroin?

Thankfully I have no idea what the market value of heroin is. Assuming Vincent was buying really good stuff and Lance might have been giving him a bit of deal on it, $300 might have been a reasonable price for that amount of heroin.

$5 would have been a bit pricey for a normal milkshake back then. Vincent did admit it was really good, but not worth $5.

Not to nitpick or anything but it was 3000 worth of H not 300. But yeah that does make his milkshake complaint look more stupid.

here's one more thing i love about QT..
he writes dialogue that most people agree is meaningless, yet here we are 26 years later debating the $5 milkshake

: )

because drug addicts do these things, the money spent on drugs does not count