Application note from CREE on efficiently designing a conventional FR4 PCB to manage thermal heat and as a cheap alternative to metal core pcbs. Link here (PDF)
One of the most critical design parameters for an LED illumination system is the system’s ability to draw heat away from the LED junction. High operating temperatures at the LED junction adversely affect the performance of LEDs, resulting in decreased light output and lifetime. To properly manage this heat, specific practices should be followed in the design, assembly and operation of LEDs in lighting applications.