Qimage can print multi-page posters.
http://www.ddisoftware.com/qimage/feature.htm
free download and evaluation.
Depending on how big you print, and how may pages it would take to assemble your poster, paper and ink could add up to be expensive. I suppose that if up close quality is not an issue, you could print on cheap photo paper and at less than maximum quality to preserve ink.
I have used Qimage for a long time for all my photo printing, but I have never attempted a poster.
hope this helps.
J