I would look at buying rock new unless you KNOW the exact history of the used rock someone is trying to sell you. The last thing you want in a new tank is a pile of rock with nasty critters in it that the seller couldn't get rid of himself.
Live rock $$$$$$
Dead rock $$$
Bare tank -
I would buy new sand just because there is other ways of getting a tank started than using used sand from someone else.
Measure once, measure twice, and measure one more time before you whip out the card and buy something that will not fit your tank.
Look at things that you can create yourself. RO water can be bought but spending a little up front for a RO kit so you can make your own will pay off in the end.
It seems to me that most of the money does not go into what is in the tank but into what is needed to keep alive what is in the tank.