The problem is that I had to get the cheapest one to be able to add it to the tax writeoff in a single year, otherwise it's split among three years (the tax code in my country is weird like that). If I could aftermarket-upgrade the RAM, I could just have bought the basic model and then upgraded, but that's not what Apple wants me to do.
Also, how should I know that I needed more RAM? It's not like they're writing that on the box (quite the opposite actually, Apple says that 8GB is enough for everyone).