If you want something thats cheap and half the price of a GFX go with the LRP Pulsar Competition 1 or 2. They have linear/flex charging, along with 10amp discharge (could be higher), motor run in. Its also very small about the size of your hand or so. I think the LRP is the best charger for someone looking for something not that cheap but effective.
dont get me wrong the GFX is a sweeet charger, just sometimse its alot to carry around, but its worth it most the time.
Jon