can someone tell me why the item that gets dropped if you die is the most expensive, or is that the reason itself, because they are the item that drops if you die

That's the reason they are expensive, they are the most in demand because of dying.

Isn't it a rarer drop than other barrows items as well?

I'm sure, I highly doubt it would cost more than the other items combined if the only reason is you dropped it.

all barrows items drop equally rarely. The prices in the market is dependent mainly on the usefulness of the whole set, then which piece is easiest to lose - ie weapon, helm, legs, plate

I don't think that's entirely true seeing that Guthan's Helmet is so much more expensive than the legs and plate. That may make sense for Guthan's Spear though, alone only.

street price has nothing to do with its value when you die

You keep the top 3 items in your inventory with the highest High Alchemy value.

no its not the alch value, its the value you sell it at a genral store

 

i.e you buy a whip from a player at 1.8m and you vaule it at a genral store its only worth some like 48k, so if you die it's worth more by that then at the price u bought it at. it's like the value of black cav its about 84 gp at genral but players sell at 1m-1.2m. etc etc