Buy a mooring buoy, some chain and a shackle large enough that it wont slip through the middle of the buoy. Attach the shackle to one end of the chain. Thread the non shackle end of the chain through the buoy and attach to weight.
I cemented my chain inside a discarded tire that filled with cement. To do this cut a circle of plywood and put inside one side of the tire. Fill with cement and embed chain in the cement. Might put a couple of bolts through the chain links in the cement so it grabs in the cement better.
Once the cement set I tied a three person tube to the top of tire and rolled the whole thing into the lake. Float it out to where you want it and cut the rope attaching it to the tube. Viola!
Cost wise you are out the mooring buoy, chain, shackle and cement