Going rate for Aircon maintenance

Hi guys the time has come to clean out our Aircon unit's although it's the first time we've done so. Have asked some friends and they have referred people quoting $500-700 cleaning per unit. With my flat that would be 5 units at minimum $2.5k going by the lowest rate, does this sound about right? I'm in the tsuen wan area, would welcome any any referrals of reliable workmen, thank you