I think monthly or annual is actually a better option if money is a concern. From what others have mentioned on the site there’s been a holiday sale on lifetime the past few years so it might be worthwhile to wait before taking that plunge. In terms of raw numbers most users seem to drop out around level 10 or 20 as well. If you aren’t sure if it’s worthwhile it might be better to do a non-committal subscription until you’re really sure about it. Besides, the annual sub is prorated if you upgrade.
As for WK itself it’s fantastic and I have no regrets. There’s no way I would have made as much progress with kanji on my own in such a short time. Wait, actually I have one regret. WK is a soul sucking addiction I can’t ignore in my Japanese studies.