lets take a look at statistics shall we mane!?
First Carnage-$3.00
First Venom- $85.00
Ok so the second one isn't bad, but lets take a look at the first one... (which is newer then the second one to be fair...)
How can one of Spidermans most popular villians only be worth $3.00?? Simple... The comic was over printed, making supply greater then demand... basically we have a trillian copies of this issue, making it worthless. You could probably pick this up on ebay from neatstuff collectables for $1.00 ($.01 for the comic, $1.00 shipping) Personally I think neatstuffcollectables is horrible on shipping prices, but still, it's not a million dollar comic book, and you will never get rich off it, even if you wait 30-40 years.
Supply and demand make the comic prices rise and fall... That is the basics of making money off this hobby, not buying first appearences.
First appearences now are virtually worthless...