Walmart has a good deal on the Stanley (SS4LS) Simple Start 8000 maH Li-Ion Jump Starter for $29.98 + free store pickup!
A bit about this item:
The Stanley 8000 maH Li-Ion Jump Starter and Mobile Power Pack is ideal to use for an emergency jump start and also for charging portable electronics on the go. The 8000 mAh lithium ion battery can be charged 2 ways either in home via an included AC plug or in the car via an included DC plug. It also can charge and extend the run time of all your electronic devices such as phones, tablets, laptops etc. via its dual 3.1Amp USB ports.