Todays anniversary deal for example isn't too bad. It's a 10+1 roll and then a pick-your-rainbow from a selection of Sakura, Black Baccara, Cactace, Mei or Alpinia for 3,000 Nutaku gold.
F2P players can seriously totally forget about ever obtaining all rainbows (on avarage, you'll get one once every 18 10+1 pulls), and this is about as cheap as Nutaku puts them on offer. The usual price is 6,000 NGold.