Are '73s really worth this much?!

[Number21]

I was just given a nice '73 K20 for my birthday, I don't know how much it cost. I've had several '79s in the past but they were never worth more than a few hundred bucks.

Anyway, I'm trying to figure out how much I should insure it for, so I looked up the prices from NADA. They say it's worth $4200 "low retail" and $8800 "average retail", I'd say it falls somewhere in between there condition wise. That's a HELL of a lot more than I thought it would be! I compared it to my '79 and it's only worth $2000-4000 according to NADA.

Is the '73 really worth that much of a premium over a later year? I know 73/74 are a bit unique...but then again so is '79. I mean, if I had a '73 and a '79 in the same condition, would the '73 actually be worth nearly double?! I don't understand...