Amazon Wireless drops AT&T Lumia 920 price to $49 on contract. Colors still backordered.

I don't think Google helps anyone who makes android phones. So OEMs have to sell what they feel will make them money after paying Microsoft/Apple and whoever patents android infringes on. Don't think that androids don't get discounted, only Galaxy S III, and iPhones don't get discounted but again it depends on carriers. For example, Sprint have been selling iPhones at least $50 cheaper than apple stores so apple stores did price match. Best Buy is giving $100 gift card if you buy Galaxy S III from Sprint. So don't think android and iPhones don't get price cut. Its just media doesn't overblows it like it does in Nokia's case.
Now for Lumia 920/820, AT&T/Microsoft/Nokia are subsidizing it. Don't believe me then checkout the price of Lumia 810, its $150 from T-Mobile, Costco is selling it at $129 and so does Walmart. I don't remember Amazon pricing for it. So the conclusion is, carriers have a lot of room to cut the prices and then retailers can do extra price cut to boost sales.
I am not an expert but I am telling my observations.

There are a lot of things at play here that may not make sense if you aren't in the U.S. market.  Because we're mostly just retarded here but hey... it's how we play. 
To start with there's a HUGE push to be the company that sells the MOST of something.  So a company like Walmart might undercut the price on the phone to get more people to buy it.  (To them it's not the sale of the phone that matters.  They get a kick back for signing you to a 2 year contract with the provider.)  Not to be outdone Amazon will cut their prices to match.  Both do it mostly to drive more customers to their stores and hopefully generate sales on other high margin items like accessories. 
Next up is "Black Friday" which is mostly just the previous point on steroids.  As messed up as this is, most retailers in the US opperate at a loss for most of the year.  The largest single day of sales is the Friday after the Thanksgiving holiday (which is this Thursday)  This is the day most stores actually begin turning a profit on the year.  So, they'll take a couple of high demand items and sell them for some asininely low price.  A 46" 1080p Samsung TV for $126 is one of the rumored sales in my area.  Each store will have a "limited" supply of those super low devices.  People will rush the stores to get the crazy deals... and while they're there buying last years overstocked TV for less than cost the hope is they'll buy some other shit too.  I mean hey, you got a killer deal on that TV... you should buy a Blu Ray player, it's on "sale" for $60.  And since you got that of course you'll need an HDMI cable but we can bundle that for an extra $30... saves you a total of $45 on the two!  And of COURSE you'll need some movies.... a special deal at 5 for $100.  You save another $50!  (But you gotta pick from these movies you've never heard of here...)  Hey... you know... you just got a TV for a SCREAMING deal and we bundled all this other stuff... you maybe should look at a home theater system to go with!  Of course... they're on SALE!  Did you need a microwave for the popcorn you'll eat while you watch the movies? 
Lastly, Microsoft is really seriously trying to make a move on market share in the US.  (We're not by any means the largest market but it's a psychological thing.  Basically we want to think we're the most important market)  So they'll subsidise the hell out of these things and basically eat the loss Nokia and the retailers take on these silly prices.  The Samsung Note II is $299... the Lumia 920 is $50.  For a lot of folks that $50 will get them in the door.  And once you get a customer on your plan they're generally hooked for life.  There's not a lot of mobility in the smartphone market... If you're an iPhone guy you're not switching.  Android folks buy Android.  The consumer who does switch is fairly rare.  So getting the new kids, and some switchers based on price alone helps.  If they can get a market share they can claim some sort of validity to their OS.  Then they can start to compete head to head. 
So seeing prices slashed so quickly isn't really that surprising.  The phone is selling out pretty much everywhere which would indicate a high demand.  (Mine shipped today... only 6 days late!)  Given the nature of the sales weekend here in the U.S. it's no surprise the stores cut their prices this fast.  Basically they'll cut a bunch now in the hopes that you and I buy today instead of waiting for the crazy sale on Black Friday.  So a 50% sale this week leading into a one time sale of 99% off on Friday  (if the $.99 rumors are true for Amazon.)  
Does it make sense?  Hell no.  But what does the U.S. do that does make sense?   

Its AT&T doing the subsidy. Not Nokia. That is the price AT&T paid for getting the 920 exclusivity in US. Nokia gets about the same amount of money per device, regardless where in the world and to which retailer they sell Lumia 920. AT&T is probably paying about 250-300$ per device. Why would they do it? They saw potential in 920, and now hope people will make new contracts and switch from other carriers to AT&T because of the phone.

I just replied to your other post with a small book but one thing you asked here:  Wouldn't $89 be enough?  No.  At least here we like our figures in "normal" increments.  So $1, $5, $10, $20, $25 (sometimes) $50, $100 and then increasing in $50 amounts. 
A price of $89 just makes the US consumer uncomfortable.  Especially when everything else is either $50 or $100 in the same market.  People will see a $90 price tag for the phone as an "inferior phone" but $50 price tag as a "great sale" 
Sales are emotion driven, not logic driven. 

Tele Finland is selling at 480€ + 24*0.67€ = 496€, unlocked and sim free. Pre orders are sold out, but the price will be the same after the release day. Also they have nice offer for 2 years unlimited maximum speed data plan for 10e per month.

Ah, ok then. So much for that, I guess.