I saw a writer's comment on another blog yesterday where they mentioned that a client may have paid very little, but it was OK because the people there were nice (paraphrasing a bit). All I could think was "so what if they're nice?"
Look folks--it's great when you have a good, nice client on board. Heck, I hope ALL of your clients are nice to you (if they disrespect you, why would you keep working with them anyway?).
But being nice is not a substitute for paying you! Niceties don't pay your rent or mortgage. They don't put food on your table. They don't help contribute to the kids' college funds. They do nothing but make you feel all warm and fuzzy inside, and that just isn't enough.
You don't need to take very low-paying gigs to build portfolio pieces. If that's the goal, you're better off investing the time in writing "for yourself."
The only good excuse to write for very low rates is if you're using something solely for marketing (for example, if you treat AC as you would an article directory, you might get paid, but that' not the sole purpose for doing it). Another example would be writing a guest post on a reputable blog in your niche that would bring in traffic to promote your own site. But there's a huge difference between things like that and writing for "clients" (in quotes because they barely qualify as such). Clients are the people you market to--they're not the ones who give you the marketing platform. They're the ones you have to build an image for--if writing for them doesn't contribute to the ultimate image you're working towards portraying, you should spend your time elsewhere.
Even good writers undervalue themselves from time to time, but I don't know a single professional freelancer willing to work for an atta boy. If someone's nice to deal with, that's fantastic! Add them to your network. But don't waste your time on them as a client unless they're willing to adequately compensate you (or unless there's a hell of a good marketing reason to do it--there usually isn't).