Rusty! I am glad you are honest about your budget. Here is my honest opinion about why I chose SEOmoz when I did.
Before looking for SEO tool, I didn't know what was working. All I knew at the time was that I was growing my SEO. I really didn't know how to keep track of it. One day I felt that I needed SEO Tools to keep track of my SEO campaigns. I knew Google Analytics was not the answer. So I turned to Aaron Wall's SEObook and I was a subscriber there. After several months I outgrew that community "learning" about SEO. Needless to say, I canceled my monthly subscription there. I had looked at SEOmoz, but I didn't join because I wasn't ready. I thought I was, but my income was too low and I didn't need to add another bill.
There came a point when I really felt that I needed SEO tools, not crappy free ones. I needed a dashboard to tell me what my campaigns were doing AND how I was doing against my competitors. ALSO, I used SEOmoz's ranking reports to land a client. I told them we can "see" if your site is growing or shrinking.
You may be too early in your SEO career to justify the monthly subscription. I would suggest to find someone to split the cost with. You make the payment and have 3 of the 5 campaigns. Also $100/mo can be large for someone. If it was a one-time deal, no biggie. But if a man needs to put food on the table for his wife and children, he shouldn't sign up. If you feel you are redlining your finances, don't sign up. "IF" you do sign up, you better use it as a SOLID excuse to go land some real clients.