Same as Kevin, we rented our house in Çaliş through Palmiye and paid the equivelant of 3 months rent up front. I month as a "one off" agents fee, 1 month as security deposit to landlord and first months' rent.
We have recently renewed the contract on the same house, Palmiye drew up the new contract with no further charges. On the question of annual rent increases Kevin, it is written in our contract that the rent will increase by 10% each year, this seems to be the accepted thing here and with the state of the lira at the moment it's not too bad.
In the case of the OP £500 as an agents fee is rather OTT on an annual rent of £3,500.