Its not just about the hours. I work as an FI for a charter organisation and get the occasional trip as 'pilot assistant' in a Beech 200 King Air...a turboprop. This requires a type rating to log any time. But as a pilot assistant I gain valuable experience of international airports (Schipol, Dublin,) and operations. All good experience...but not logable. It does however go on my CV in an attempt to raise me above the rest (not working so far!)
I did go as far as asking my boss if 'I paid for a B200 type rating would he give me a job'. He said the problem is that the operation is single pilot so I would not only need the minimum hours described above, but also real multi turbine time to satisfy the insurance requirements (500 was mentioned).
So it is chicken and egg. You need to join an airline to get the multiturbine time....but the airlines wont have me as I have not enough twin time! Only way to serious charter work is via the airlines.
Air taxi on light twins (401, 406 etc) is different, but the insurers would still want to see a decent amount of time on twins before letting you loose as P1 on public transport.
My own opinion is that air taxi is not the game it was ...and is certainly not an easy route into the airlines as taxi jobs would appear to be rarer than airline jobs! The good news? Many of these jobs are taken by airline pilots who are tired of the routine of scheduled flying....thus creating jobs in the airlines. Well that's the theory....