I am currently making a king size quilt for my son and daughter-in-law. I estimate that the materials (fabrics, batting, not even counting thread) alone are between $250 and $300. I estimate the longarmer will charge me about $300. So the quilt is expensive even without any charge for my labor (of course I'm not charging my son, but just to give an estimate).
I'm sure you could find cheaper fabrics (mine are Moda)...