I usually rent an apartment. You may actually consider that if you are staying there for a week. There is a huge variety. I used buenos aires habitat and they were great. Have stayed twice in their apartments. Great location, good prices, excellent service from reservation to check in and check out. the apartments look exactly like what you see on line.
Some time ago I stayed at one of the Kempinsky hotels in Recoleta on Talcahuano. Very good location and I remember the price was less than 100 US$. The place is a little dated, used to be very elegant and could use some remodeling but it is clean, very good service, nice breakfast and even the little restaurant was pretty good.