Is there any reason you want to rent a serviced apartment in Manly?
I agree, it is quite expensive especially over summer.
If you are wanting to rent a place there just to see if you will like it. You don't need to worry about that because you will like it. Manly has got so much going for it and the beach is one of the best in Sydney.
So, I agree. Try and find something in a neighbouring suburb that will be cheaper.
You could also check out Manly Vale or Seaforth.
Another suburb you can suss out is Cremorne. It's about 10 minutes away from Manly by car, and about 20 minutes by bus. It's a fair bit closer to the city, but it has self contained apartments that are good. I've stayed in them before and they are clean and very convenient. There's a supermarket in the same complex, an art house cinema directly across the road, and a bus stop out the front door. The place is called The Concierge Apartments.
www.theoaksgroup.com.au