About $25 for a 2GB stick from newegg. The 1GB sticks are about $16. I would do the full 4GB to get the added benefit of dual-channel in the last 1GB for $9.
Remember if you have a Macbook that wasn't a Late 2007 Santa Rosa or later the RAM limit is 3 GB, or 2 GB of dual channel matching RAM.
Rule of Thumb: If macbook has Intel X3100 or Nvida Graphics it can use up to 4 GB
But if it has Intel GMA 950 if can only do 2 or 3.
The original Core Solo Macbook from 2006 can only do 2 gigs.