While I can't say specific to the 2.0l, the 1.6l 16v is known to have alot of egr related issues & throws the p0400 code at the drop of a hat. Mine threw it 3 weeks into owning it.

The good news is, it's 95% a cleaning issue & not parts. I agree, for a simple vacuum actualted valve, the part is ABSURDLY expensive. On the 1.6 it's almost always a matter of cleaning a tube & port into the intake. I cleaned the tube & port & it hasn't thrown the code since!!

I know this won't be the same setup as the 2l but, are you sure the valve itself is actually bad? Ie, is the diaphram worn/cracked/stuck or otherwise damaged? You can test this with a hand vacuum pump on the 1.6. I'd check & clean the path that the exhaust actually travels through before throwing parts at it, good chance it's just clogged up & won't need anything but cleaning.
As far as used parts, you could look for one from a 1.8l or 2l from say an Esteem or Swift, or a 1.8 Sidekick Sport... I believe they are same block & theres a good chance alot of parts would interchange... but don't quote me on that. Hth.
Fuzz