Pollyfilla Basing!

First off, get yourself some fast drying pollyfilla:

http://www.binbin.net/compare/Polycell-Weatherproof-Filler-Tube-330gm-Single.htm

This stuff is normally used to patch up cracks in walls, but is perfect for a flexible and low cost basing material.

1)Spread your pollyfilla on a flat, non-stick surface. Spread and thin or as thick as you like. I normally try to spread it and thinly as I can and build up several layers on a base.
2) Once dry, crack the pollyfilla.
3) Glue to a base
4) Because of the properties of the dried pollyfilla, it can be cut, filed, broken and will leave some very sharp rocky edges. It is also perfect to drill into and pin models onto the base.
5) And voila. A finished base ready to be painted.

The bases are great for display pieces, but any jutting rocks (like Cynath's base) will be quite fragile and will break with regular aggressive play. A thin layer of superglue should help this, but treat these bases with care.