用DDR謎比DDR2貴很多？

YES, DDR is more expensive than DDR2 RAM mainly because of the mass production of DDR2 RAM.

BUT you cannot use DDR2 RAM on a motherboard only using DDR RAM.
ALSO you cannot use DDR RAM on a motherboard only using DDR2 RAM.
Their voltage and contact-pin are different.

If you want cheaper DDR ram, then look for second-hand ones. Much cheaper.