I will be staying in my fully equipped Tokyo office, with bath and kitchen

of course with bed
Toyoko-inn definitely has room in the 7000yen range, their single room in Tokyo is around 7000 yen per night
The closest one to Roppongi is
http://www.toyoko-inn.com/e_hotel/00103/index.html
The two in Ikebukuro are
http://www.toyoko-inn.com/e_hotel/00096/index.html
and
http://www.toyoko-inn.com/e_hotel/00007/index.html
For the b ikebukuro I saw Standard Single (Non smoking) @ 25,800 for 25/6 check in, 28/6 checkout
Hotel Villa Fountaine give me a long list of selection, their Roppongi properties is of course expensive but their Nihonbashi one is nicely priced.