NEW:
In most cases, glass is always less expensive...A brand new 7ft glass tank in my area(San Francisco) would cost me $700-800....Plexi would almost be twice as much, around $1,200-1,500....
OLD:
A far as used goes....I have seen tanks low as $150 for a glass 7ft....Average is $1 something or $2 a gallon for a glass....Plexi will be around $400, if scratched or self drilled....Most plexi owners take care of their tanks, so price will be a couple(litterly) hundred less than paying for a brand new one....
WITH SUPPLIES,STAND,ACCESORIES:
New stands could add up to $300-500 to the sticker price....With big tanks, it is wise to make your own stands out 2x4's and cinder blocks....Drop a board(painted or not) over the front...Looks clean...