You can spend an arm and a leg on various flocks and powders and sand and stuff for texturizing figure bases, or you can spend nothing at all for as much stuff as you need to keep covering bases until the sun goes out and the universe descends into entropic stasis.
Go to any cabinetmaker or carpenter and ask them if you can have some MDF sawdust. They will probably look at you like you're crazy, but never mind about that.
MDF, when it is sawn, disintegrates into a fine, slightly clumpy powder that is perfect for texturing figure bases, and it doesn't look as out-of-scale as sand usually does. It can be coloured just by stirring some paint through it, letting it dry and rubbing it through a sieve, but its natural creamy-tan colour is pretty good for representing sand and dust.
I've coloured the MDF dust green, but that's all I've done with it, and I haven't contoured the base at all -- just glued the MDF dust straight on to a flat, painted bit of 3mm MDF. The variable sizes of the dust clumps make decent small surface rocks. If you wanted more depth and/or variation in colour, you could seal, wash and dry-brush it as well.