The price for cleaning is varied from HK$200~HK$350 depends on the HP OR BTU/hr of that Air Conditioner.
If you can speak Cantonese, try to find the local repair shop to give a good quote.
And, you said some with quite a bit noisier. I think it may be caused by the wore FAN Bearing after long time use OR the supension of Compressor.
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I just searched this site for your price reference:
http://hongkong.ads28.com/big5/Item/.../ViewItem.html
They claim you can have 15% discount on the listed price OR 20% for cleansing 3 units.