I have a similar length run to the rear speakers in my HT system. Since the rear speakers are for fill, and in most cases are reproducing a somewhat restricted frequency range, you don't need to spend buckets of money on speaker cable. Monster Cable makes a perfectly satisfactory range of cables for $1 - $1.50 per foot, with several that are made for runs underneath the floor. If you want to get a little more upscale, my other recommendation is Kimber Kable 4VS ($3 per foot) or 4TC ($7 per foot).
