I just recently shipped a saddle via USPS Parcel Post and with insurance, it was ~$40. It will vary depending on the carrier you choose and the "extras" you add on. Anywhere from $35-50 would sound reasonable to me.
Riding: The art of keeping a horse between you and the ground. - Author Unknown
Won't it depend on how heavy it is and how far you're shipping it?
I've shiipped saddles around the south for anywhere from $25 to $50 (recently sent one via UPS to VA insured for $700 for $35). I don't think I've ever paid over $50 even with having the store pack the saddle and buying lots of insurance.