Edit: I think I have to revise quite a bit. One thing is that the Peloponnesian Wars went through various stages which themselves got different titles (Ionian, Corinthian, etc).
As a result the poster on Yahoo Answers may be quite right. In the Ionian War, which crushed Athens, the Persians provided the gold for the Spartan Fleet.
In the Corinthian War, which pitted Sparta against former allies, Persia turned around and financed the other side.
Initially the Great King, Artaxerxes II, funded this
anti-Spartan alliance, 49 because the Spartans had abandoned the
treaty which they had struck with him during the Ionian War.
Instead of letting him levy phoros on Ionia’s Greeks the
Spartans were now fighting him for control of them. Athens
used Persia’s gold to rebuild its fortifications and its fleet.
Which Persia didn't like one bit, so they changed tack again:
These Athenian actions were manifestly at Persia’s expense. By the early 380s Athens was even backing revolts
against the Persian Empire in Cyprus and Egypt. Artaxerxes
II thus realised that by helping Athens to fight Sparta he
was fighting fire with fire. The Athenians were now a bigger threat
t o his empire than the Spartans would be. Therefore the Great
King agreed to support Spar ta financially as long as he got
complete control of Ionia’s Greeks. With Persia’s financial
support the Spartans assembled and manned quickly 80 warships
and sailed to the Dardanelles where they stopped the grain
ships sailing to Athens. This action brought the Corinthian War to
a speedy end. The Athenian d ē mos feared being starved into
submission as they had i n
405. Consequently when Persia summoned to Sardis al l those who wished to hear the general peace-treaty which its king
wanted, the amb assadors of Sparta and the anti- Spartan
alliance arrived with flattering speed.
This paper offers an interesting insight: It wasn't just the value of the Persian gold, but also the simple fact of the existence of Persian coinage kept the wars going. It seems that only Persia had a silver coinage that was useful to be actually used as means of payment, including to be carried about!
Sparta of course did not strike coins until the third century. (Financing the Peloponnesian War: the Peloponnesian perspective, Jennifer Warren, page 317)
Otherwise the city states used a crude form of bullion, which was quite impractical for anything but hoarding wealth:
Hodkinson in his wide-ranging exploration of Spartan wealth as such
refers to Spartans stockpiling foreign currencies or sending them for
safekeeping to Arkadians (ibid.)
By the end of all this, with everybody lying about exhausted, this backwater named Macedonia comes about and sweeps up the lot.
Including the Persians! ;)