There used to be methods to get cheap codes through promotions or upgrade accounts but not really any more. As of now, nobody really sells the codes and your best bet is to see if you can get spotify student (just spotify with 50% off) if the promotion is still running. There are also plenty of account sellers around, but the account likely is going to be shared and seeing as you want to keep your playlists, I would recommend against this.
Thanks for the advise. Guess I'll just have to buy the old fashioned way!