Wha's the cheapest way to reach Xiamen from either hong kong or macau?

The price is hard to tell, because if you book in advance, you can get a discount from the airline companies.
As I know, the Xiamen Airlines, the Southern Airlines, Cathay Pacific Airways, Dragonair have direct flights between HK and XM. The XM Airlines, Macau Airlines, Southern Airlines and Air China operate the buses between XM and Macau. It is suggested to look more price issue online to choose the cheapest one.

Personally speaking, flying from HK is suggested.