I started to use plastic bases but found they often fell over, especially with plastic figures, or wouldn't grip on a sloped surface. Now I use exclusively coins (2p or 1p) to mount single foot figures, MDF for horse or crew figures and multiples. If I want to group single figures for movement I use the appropriate sabot bases, usually by Warbases.
The coins give heft, especially to plastic figures and they are literally the cheapest way to mount figures (and that includes washers).