That is still a decent price difference, especially considering that unless specialised vendor-specific tools were required I would never pay for an adjustment or strap change. Anyway, is it mentioned on Amazon.com that they are not authorised?
If $18 is a "decent" price difference (leaving aside the increased cost for 2-day shipping) then that's up to you.
As for being a gray market watch, it is mentioned in the user reviews for the Black Monster (stainless strap version) and in this discussion thread on fatwallet.com:
"Now on to the watch, I noticed that it did come with the blue warranty card, but there is an insert from Amazon that says they are not an authorized Seiko dealer, so the warranty is through Amazon for only 2 years."
I've never bought a Seiko from Amazon, FWIW. However, I have purchased from Bluedial and they are not gray market. (I'm not affiliated with Bluedial in any way.) I also don't own a Monster. But to me, I'd rather pay a little extra and have the peace of mind (in addition to having the band adjusted ... I'm not Mr. Fixit like some of you all haha).