The east coast USA has a heap of fishing reefs (-20m+) built from discarded tyres during the 70's, not sure they still do it, concrete appears the prefered option now, do a google on reef balls. Essentially, the material cost is less an issue than transport, installation, management and study costs.
The concept behind the sand slug project i worked on at Nth Cronulla - was to create a temporary sand-only bombora at -4m to -8m deep. Some fantastic peak formations formed up in the lee of the feature. Yes, the sand dispersed shoreward, especially with a big storm on 1st August, and built up the beach. Key issue in this project - it cost no extra dollars to improve the surf. So i expect we will see more projects like this.
Here is an overview of the Perth Reef
http://www.surfingramps.com.au/CablesArtificialSurfingReefHere is an overview of the Narrowneck Reef
www.surfingramps.com.au/NarrowneckHere is an overview of crew up near Bundaberg (small waves, big tides), who got out there at low tide with an excavator and shifted the boulders, another low cost method.
www.surfingramps.com.au/bargarasurf