Berceni is the area where a guy killed a friend right in the front of the Mall some months ago. See the history here: Portretul robot al ucigasului din Berceni: ZIUA (romanian only).

Berceni is one of the worst areas in Bucharest, being much better however than Ferentari, which is a 'no country for old men' area. But speaking of civilized areas, Berceni has the worst prices / sqm in Bucharest and it is far away from the Bucharest's centre.

You really seem totally strange of your investment. Before asking the real estate agent about prices you should ask yourself if you ever knew what have you bought. But yes, a good agent should have been able to rent it on your behalf for 300 euros. However do not forget that there is a big crisis now in Romania and many agents are selling anything, anywhere, no matter how, just to make some few bucks. Maybe your agent didn't had enough time to wait until a 300 euros / month client appears.