to clarify, PC2-4200 was the ram used in the final revision of the powerbooks. PC2-5300 is used in all the intel macs.
but they are compatible. i used pc2-4200 in my imac when i didn't want to spend more money on buying completely new ram.
that being said, i think $80 is a little on the high side. that might very close to the prices at best buy, but newegg sells the pc5300 in that range