Last fall I started a "crusade" to make afgans for each of my three stepsons, my godmother and three nephews. Right now I'm finishing the sixth in the series (plus baby wraps and blankets for the family new arrivals!). For four of the afgans I used Caron's "Simply Soft", a beautiful synthetic yarn available in 7 oz skeins, very economical. (under $4 each and it takes me about 6 skeins to make up the patterns I'm using for each afghan). For a "napghan" you probably wouldn't use more than 4.
It's a beautiful yarn, washes well and super soft. I love working with it and the recipients are thrilled with how soft and warm it is.
One of the other afghans was made with Lion Brand homespun, which is more expensive but very soft and beautifully warm! It, too, is synthetic.
Good luck with your project!