i would agree with what amy and lou said a working flat is a big investment especially if its a second one simply to work from then to make it worth while you would have to really commit your self to working pretty much full time in and most likely daytimes rather than weekends
i dont know where your based but i know where i live in Bristol my last flat that i rented in 2006 was a large two bed two bathroomed full furnished in a block with intercom ect and through an agency cost me 750 a month plus bills and i have no idea what it would cost in somewhere like london probably heart stopping i would imagine
initially with agency fees it cost me first months rent of 750 plus a six week deposit of 1050 i think and 200 agency fee /credit check so around 2k and that was in the days when escorting was a licence to print money as it was really busy and prices were a lot higher
so my weekly cost were alone were 175 just in rent alone plus bills ect so convservativly i reckon at least 25o a week and that before i have even spent a penny on advertising
and then on top of that you may have to furnish it as well with beds bedding curtains ect i didnt with mine but there is always stuff that you will need to make it comfortable if your spending a long time there
However if your able to rent a place to live and also work from at the same time then i think it become a lot more cost effective to the point that you can get something that you really want as your going to be living and working from the same place so only have one set of bills to pay
With regards to paying for advertising its really is a case of suck it and see
for some particular types of adverts work fine and then dont work as well for others
populars sites like punternet punterlink and local regional directories and fourums and also having your own website as well should in crease your business and not just relying on adultwork
personally if i was going to rent a flat again to work from i think i would have to advertise in the local papers to ensure that i stayed busy enough to warrent the cost of a working flat (which i did for a long time) but i dont think i could go back to the endless question and timewaster that come from the newspapers