Only telling us that 'an MTX Class D amp' is included in the package doesn't tell us heaps. What model is the amp?
If this person offering you this deal has any ability as an installer I would say it's likely worth close to $400 to the average person. MTX's 6000-series subs aren't their best...but they aren't the worst model that MTX sells either.
Provided that (1) the amp is capable of driving the subs adequately (2) the box is suitable for the subs [can't just throw subs in any ole box and have it sound good, dontcha know..] and (3) the install you get from him is right...meaning things will be wired correctly, there won't be any wires hanging around unsecured, settings on the amp are adjusted properly, etc....
I'd say $400 is a decent price.
Still like to know exactly which amp is coming with that, however....