Was the fair market value of a kid goat really two zuzim? This piyyut is of course allegorical, but, if the value given is highly inaccurate, is the discrepancy in value symbolic beyond the symbolism of the value itself?
A colleague of mine reminded me that there is actually a Gemara in Berachos 44b that describes a young healthy goat as a 'bar zuza', meaning it costs one zuz. He explained that although Chad Gadya states that the goat was bought for 2 zuz, there are major commentators (see Haggados of the Vlna Gaon and Chasam Sofer) that explain that the repetition of "Chad Gadya, Chad Gadya" means to hint that the song is actually referring to two goats. This would fit very neatly with the price of 2 zuz. As to the deeper reference and meaning of the two goats, see the above-mentioned haggados.
Rabbi Jonathan Sacks says the two zuz are Moshe and Aharon who Hashem used to redeem Bnai Yisrael.