I personally put as low a figure as I think I can get away with - the gubbermint doesn't need my money anywhere NEAR as much as I need it.
If it were defrauding a person, then yes, it would be unethical - but sit there and honestly say you get value out of the taxes you pay?
So it really is a decision you need to make yourself - do you think it reasonable to expect to have gotten a roadworthy bike for $2000?
In America, I think it would be. (the land of the low wages and cheap stuff)
Cheers,
Rick
EDIT: do you not need a receipt or bill of sale? I usually just get the seller to fudge the selling price on the receipt!