Saruman's economic infrastructure unexplained?

[Tuor of Gondolin]

I believe somewhere (in Letters?) Tolkien makes
observations regarding sauron's slave-tended fields in Nurn
something to the effect that even a tyrant needs an
economic infrastructure to support his policies. And it seems
that other areas, including Gondor, Rohan, Hobbiton, Lorien,
Laketown, the Beornings, etc., all have either an implicit or
explicit economic infrastructure. The exception seems to be
Saruman. Are there any suggestions or deductions as to how
he could support a 10,000 + army and some hundreds of auxiliaries,
such as wargs and the nonfighting elements needed to supply
them? After all, he controlled a fairly constrained area, the
Dunlendings don't seem to be more then a subsistence economy.
And he was hemmed in by Fangorn, Rohan, and the virtually desert
areas of Enedwaith. You can see how Sauron could draw on his
slave fields and tribute from east and south, Gondor on its outlying
areas, including the Pelennor Fields and South Gondor. But where
was Saruman's economic basis for power?