Bison,
The Carbide teeth on your Fecon should give you a good 300-500 hours of solid performance. Many people claim much longer. Although they are not meant to grind rock they hold up very well in rock. Of course avoid as much rock as possible but the unavoidable stuff usually does not hurt the Fecon teeth at all.
The carbides are not really replaceble, although I have heard of people trying to do so. However, if you do loose a carbide, you can continue to grind with the tool as long as you are not wearing into the tool holder behind it without hurting anything.
The black head you mention makes me think it is a Fecon built ASV branded head. If this is the case you should have 45 tools. If so and you get 300-500 hours out of the tools then the tools are costing you about $6.75-11.25/hour to run. For the production you get out of that head that seems pretty reasonable.
Please let us know your experience once you put it into the brush and hit those 8" rocks. I'm pretty sure you'll be impressed.