How long does each one take to complete?
I once earned a fiver by agreeing to a marketing survey. They sent it through the post in an A4 envelope and I kid you not, it was nearly 200 frickin pages with a crisp £5 Note enclosed. Took me about 3 days with half an hour here and half an hour there.
I enclosed my own A4 sheet when I returned it in the prepaid envelope....you ever send me one of these again and it goes in the bin!