Consider using a bivy sac for additional benefits.
Another out-of-the-box option is to use Refletix. I've used it on the ceiling of a popup ice fishing shelter. No reason it wouldn't work as a cost effect method to increase the R-value of a sleeping pad. Comes in various sizes and is very thin (i.e. lite weight).
http://www.homedepot.com/p/Reflecti...Insulation-BP48025/100052556?N=5yc1vZ19n
