Perhaps you could try Boku, but I don't really think any other cheaper yarn would get such good results as Noro Kureyon. Nobody WANTS to pay that much for a blanket, lol but I guess you gotta do it if you want it to look so awesome.
That's why it's such a long-term project for me. I buy them as I can. This is actually a perfect project for long-term projects on a budget - because you don't have to worry about matching colorways or dye lots. You really can buy as you can afford it