As mentioned above, if you wanted to play the card Aeromunculus, for example, you'd need to include islands and forests in your deck to give you a good chance of having access to both colors throughout the game
Currently, a Near Mint Holofoil copy will set you back about $23.13 on TCGPlayer, which isn't bad considering a massive 294 cards have already been sold since launch
As of release, Alternative Art Parasol Lady is going for around $25 to $30 , making it a relatively cheap pick-up for such a pretty card
It costs only 1 mana more, but it has the same power if you count both draw step and attack steps
Back when I was trying to put together my foil Cube in 2009 and 2010, it was easy enough to find things like Revised dual lands and Gaeas Cradle as long as I was willing to pay the price