Newegg offers a $25 Skype Prepaid Credit for $25. Coupon code "EMCWHWA23" drops it to $17. That's 32% off and the lowest total price we could find by about a buck. (We did see the same credit for $4 less during the week of Black Friday.) A redemption code will be sent to you within 48 hours of placing your order.
Also available is the $50 Skype Prepaid Credit for $32 via coupon code "EMCWHWA22", another price low by $3. Deals expire December 25.
Update: Microsoft Store has a $10 Skype Prepaid Credit for $7 with free shipping. That's $3 off and the best price we could find, although it was $2 less during Black Friday week.