Unless your websites are generating revenue, it's not worth it. Even for low competition niches, you have to spend about $50-200 a month on SEO to keep it up.
High competition niches, I've spent in the neighborhood of $10k+/month..
Super high competition, I've seen people spend $10k+/day easily.
What niche are your websites in?
PS: You have to add another 10-20% on top of the above figures for a retainer for whoever manages your SEO.