actually, it is almost nothing. at least on the tank itself.
I used to work at a LFS and on most tanks the markup is very little. because tanks are expensive to build ( glass and labour) and ship (trucking all over the country) the retailer's cost of the tanks is high already, so the mark up they charge is not much at all, otherwise tanks would cost a fortune.
for example, a 125g costs the store about $250 to buy, and they might sell it for $300. thats it. We never got our staff discount on tanks for this reason. mark them down a bit from regular price and they are below cost already.
now, all the other stuff? well, thats a different story LOL.
but the actual markup on glass tank is very small, contrary to popular belief.