In this economy, many people are struggling financially and can't afford Christmas gifts. No one should ever feel bad about not being able to afford a store-bought gift, especially those who are on a fixed income.
In my family, some people have a lot of money, some people have very little money, and some of us are somewhere in the middle. And we all know who's who. It's likely that your family members already realize you're on a fixed income and can't really afford to buy gifts for everyone. If that's the case, there's really no need for an apology or explanation in your card.
If you feel the need to say something anyway, I'd suggest keeping it simple with a phrase such as, "Although we're not able to afford gifts this year, we hope you will accept our best wishes for much peace, joy and happiness in the coming year."
And if you'd like to "give" something free, go to Smilebox and create your own personalized animated card or slideshow. Their service is completely free and you can use one of their hundreds of designs to share a favorite family recipe, write a personal wish, or share family photos. It's a fun and personal way to offer something tangible along with your best wishes.
If you decide to do that, you could include the link in your Christmas card with a phrase such as, "Although we can't afford to buy presents this year, we hope you enjoy this small gift we created online." And then you can put the link below.
Hope that helps!