The quality and service associated with Airbnbs tend to be hit and miss throughout Turkey, and Istanbul is no exception. Given the scale of the city, general availability is sub-par, compounded by the insatiable appetite for accommodation from both year-round tourists and the 15 million people who call it home.
Supply comes nowhere near meeting the high demand, so finding a good deal, especially during peak seasons, requires some persistence and a good chunk of luck.
A core contributing issue is the historic building stock in many of the popular areas around Beyoğlu. More modern commercial units are likely to be found outside of the immediate tourist areas, in the more residential neighbourhoods towards Osmanbey, Beşiktaş and Şişli.
Due to this incredibly high demand, the chances of finding an outstanding deal are close to zero. If booking via Airbnb, expect to pay around £1,200 per calendar month for a reasonable quality, renovated one-bedroom flat in a central location. New-to-market apartments come up frequently, so you may be able to grab a relative bargain on an unproven unit.
Occasionally you may see some cheaper deals in Fatih on Airbnb and especially in hotels, but if it's your first time visiting the city, I'd avoid any temptations to book in those more conservative areas to save costs.
If you are sensitive to noise, pay close attention to reviews on Airbnb regarding this, especially if staying close to Taksim. I stayed on Süslü Saksı Street on one trip and whilst it was incredibly convenient to get around the city, being routinely woken up at 3AM Thursday through Monday by a techno party in the middle of December somewhat hindered the experience.
Due to all the above, book well in advance and pay close attention to where you plan to work from. Whilst public transport is generally efficient within the city, some journeys can take far longer than you may anticipate, so plan ahead if you intend to commute across the city to a co-working space.
It's worth noting that Istanbul is situated in a highly active seismic zone and has an unfortunate history of devastating earthquakes which have caused immense damage. It's only a matter of time before the next major event occurs, and you may see newer properties carrying 'earthquake-proof' tags as features.