How AK change to RPK???

I ve'just seen the conversion kiT of RPK,some of them is very expensive!

Such as G&G,it cost $HKD 4600!!!

But,SRC RPK conversion kit just cost HKD$450!!!

Is there any differences between these two produsts?

Are these kits fit my AK47(TM)?If not,how to make these kits fit my AK-47??

Is it better to buy a finished product of RPK???

Thanks everyone!