I know WORTH is a RELATIVE term but...

Coupons usually pphf for 10% face value.

Wine Tags are closer to 30% face value.

Box Tops are worth $.10 to schools so I equal them to a $1 coupon.

But what is a fair value range for Kellogg's Flyers certificates and saving bonds certificates?

Hmmm...I don't know if this helps or not but I sell SW tickets on ebay (or rather I give them away free w/the purchase of a an empty envelope) for $300 usually which equals 1 r/t domestic ticket. I would love to start collecting miles in trades but I think that the specific airline rules on how it would transer also depends on their value. I guess I would calculate it by the miles worth and offer 30-50%. For example, one rt ticket on America West can be either 15,000 or 25,000 depending on distance. A short distance ticket for them is around $150 and long can be $300 and up depending.
Another way to look at it would be people usually earn 1 pt per dollar on their credit cards.
I don't know just thinking out loud here....