Trex 500 electric might be the largest electric to go, anything beyond the battery cost goes up very high. But do check fuel costs, starter, and other equipment. I have heard that the difference is not much depending your location. Few folks at our club have gone the A123 route (Lithium nano-phosphate) packs. Basically buy Dewalt packs, and use the cells to create own battery packs. They are very safe, quick charging (6C charge rate, so 10-12 minutes charging cycle) and cheaper than Lithium Polymer. For larger helis, this may be more economical to use than fuel or Lipo. They can almost fly continuous, as charging and use cycle time are almost equal.