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A good combination of price and cost is to use bird grit from your
local farm supply store, or buy parakeet grit at the pet store.  In
either case, you may need to do a bit of work to get it looking good,
but it is vastly less expensive than hobby shop ballast.

Parakeet grit sometimes has a yellow hue to it.  Give it a wash of
dilute india ink.  Bird grit sometimes has particles that are oversize.
Strain it using a kitchen strainer.  Prototype ballast in my area is
mostly 2-3 inch diameter, with a few pieces down to 1 inch or up to 4 inch diameter.  Therefore, most of your ballast should be 2/64" to
3/64".  I use the pieces that are too large as fill, then put the
proper size particles on top.  Cement it to the trackbed using dilute
glue or artists matte medium with a few drops of liquid dish soap as a wetting agent.  Matte medium is more expensive, but is reputed to
bestow less sounding board effect to the roadbed.  I guess which one to use depends on your pocketbook and how much clickety-clack you want to hear.

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Ted Larson