Managed to get a good deal through Hotels.com, and ended up at rougly 10 000yen a night.
APA Shingagawa was on the cheaper side of the hotels we looked at, but that had little to say on the quality. The room was a bit small, but as expected for the price.
The room was in very qood quality otherwise, with plenty of outlets for electrical equipment, airconditioner built into room, and a standalone airconditioner. The power in the room requires the keycard to be plugged in at room entrance, so usually very hot when returning, but double ac helps cool down within minutes. There's a fridge in the room also, but that is kept on a separate power line, so you can leave the room without the edibles perishing. Only negative for the room, would be the hard pillows, but that's a standard japanese thing I've come to learn, and I quickly accustomed to them.
The hotel also features an onsen, or possibly a sento, at the top floor, during most of the day, into 2am.
Other than that, there's the location of the hotel. I suspect that the price drop, is mostly to the hotel being on the outskirts of central Tokyo, but a 2-3minute walk from hotel reception to train platform more than makes up for that. It's only connected by the Asakusa line, so you should expect to switch lines for most trips, but my group usually could go anywhere by central tokyo in 20-40 minutes from our hotel.
All in all, a great hotel, and I could certainly visit here again :)