Instead of worrying about what is cheapest, perhaps you should be thinking about what is going to be convenient to get hold of and reasonably priced for your needs.
With a very slow internet connection, and without a credit card (my bank says I am too poor despite being with them for ten years, and never having ever been in debt) I put my early 2009 Mini into the shop to get 4 GB of RAM and Mountain Lion installed for cash.
They slipped 4 GB in along side the original 1 GB of RAM, so now it has 5 GB, which seems to be any amount for my humble needs. I don't know, or care the price or the brand of the memory. All up the cost was 4,000 baht (about $US 130); money well spent, I felt.