I haven't done it with Amex i don't think, but i have had several cards where i signed up, got the bonus points, even balanced transferred an amount over to get 6 months or so of near free interest, then cancelled and got part of the sign up fee pro rata'd back... Most recent one was a Westpac Earth offer netting 16,000 QFF which ended up only costing about $40 even though there was a stuff up which meant i didn't get the points and so couldn't cancel till about 6 months in instead of 3 months, so many providers will do this, just a quick ask when cancelling and they will work out how much they owe you...
I make 1/12th of $900 = $75, so yes could be cheap MR points if Amex will pro rata it back, but cards often say that you will get your points like 6 or 10 weeks or so after your spend and some deals these days are even saying you will receive your bonus sign up points in equal amounts over the first 6 months of continuous spend each month of your membership as no doubt they are cottoning on to the practice of cancelling straight after the bonus points hit the account, with or without a request for a pro rata refund... So it will probably take longer than 1 month for Amex to credit the points, more likely around 3 months...