If your serious about tumbling larger rock including agates, obsidian, jade, petrified wood.  You might want to consider building your own base equipment.  I find the commercial tumbler barrels are hard to beat for the 12 lb size.  For first rough grind however I use a homemade 100 lb barrel.  This allows me to get a lot of larger material started. The roller base is easy to make and maintain.  The most expensive part of the tumbler is the bearings.  Industrial bearing insure years of continuous efficient use.  Even using expensive bearings the cost of the tumbler base is a fraction of a commercial tumbler with cheep bearings.  Used motors can be found for $25 or less.  An electric motor running on a light load uses minimum current.  Making the tumbler run efficient will keep the power bill increase under $10 per month.   

If you want detailed plans and lessons learned we now have a DVD for sale. This DVD includes pictures of my best tumbled specimens.  It includes my recipes for obtaining mirror polish even on obsidian and clear quartz.  I show a step by step guide of how to build a 48 inch track rotary tumbler.

 Go to Tumbling Rocks the DVD in menu in the upper left.