That amount of dead rock is fine for that price...the trouble is, you need to make it "live" again in order for the rock to become your main scource of filteration. To do this, you will really need to seed the dead rock with a few bits of the live stuff (live rock goes for £300 for 20kg or much cheaper if you get it private)
You need to get all the little critters that will keep your tank going...copepods, baby brittlestarfish, amphipods, mysid shrimps and lots of other little beasties.
I have a 10 gallon nano reef and it is currently fishless until the fish i want (yellow headed jawfish) gets imported. It has about 10kg of live rock and i managed to get a few cartons of "pods" (copepods etc) off the web and also feed the tank with phytoplankton to keep it going.
Result is a stunning little piece of ocean that is full of life.
Good luck and keep asking