I have never done it myself but several others who post here have done it. Service apartments in good locations will not be much cheaper than hotels, if cheaper at all. One think that has worked well is negotiating directly with hotels. Recently the Windsor Atlantic (former Meridien) in Leme has made good deals as has the Sheraton Leblon on Avenida Niermayer. The service apartments, though, usually have hotel-like checkin-checkout too.