Are the two cores of rMPB enough? I am thinking of purchasing a 13" rMBP. It has two cores. My question is, are two cores enough ? there are other laptops with 4 cores at much cheaper rates available. How does apple seem to pull it off with only 2 cores ? will the performance of a rMPB and a quad core(similar specs) have the same performance? 

and, is it worth buying the 2 cores ?
It depends on how you're going to use it.

If your daily MacBook usage only consists of stuff like websites browsing, watching movies on VLC and creating a Word or Excel document from time to time, then, I would say, 2 cores are surely enough. 

But if you're using software that requires more resources like audio/video processing (Adobe products, for example), graphic excessive games, etc. then go for 4 cores.