Some good ideas, as an added cheap idea you might consider raiding shops immediately after Christmas to see what they do with the store decorations - many (UK at least) supermarkets just bin them on the grounds that it's more cost efficient to bin this year's and buy new next year rather than try to store them and then reuse next year. Stores that are binning decorations will usually let you take as many bits as you want, although sometimes they won't let you pick the bits you want just give you a sackful to sort at home.
Second tip - if using plaster etc for bases you can pre-colour it using either acrylic (craft acrylic or even better junior school acrylic is usually cheaper as it comes in bigger bottles than modelling acrylic) or powder paints (again junior school or Early Learning Centre etc are cost efficient). Pre coloured bases will still need painting, but if the paint chips at least you don't get the bare plaster colour shining through. On that thought, muddy brown is usually a good colour to stain the plaster.