It's not the cost. The problem is electrical planes. Helicopters are known for their agility, coming at the cost of extreme energy use (and, imho, extreme danger. Any mechanical failure in any part of the helicopter WILL result in contact with the ground in 2 minutes or less. Redundancy (in your standard helicopter craft) is effectively zero). Getting a normal plane to fly electrical is a huge challenge. Getting a helicopter to fly electrical is a bigger challenge.
In 2011, for the first time in history, a manned helicopter took flight. More than 1/4th of the flight mass was batteries and that afforded the craft ~2 minutes of powered flight (hover, not maneuvering). [1]
We just don't have the electrical storage solution to pull it off easily. It's not impossible at these efficiencies either, but it's bloody hard.
It's not just electrical power that falls short. For the longest time, planes were quite picky fuel wise and gasoline also fell short. Even today, it's not gasoline that's used, even though most planes will fly fine on your car's gasoline (not diesel though), even corn ethanol, but less efficient (range reduction of ~20-30%).
The answer to most of the "can we do X with electricity" is simply : find a way to store 4 times (or more) the electrical power in something the size and weight of our best batteries (take lithium-polymer), and it'll work. Without that, it's barely possible.
We do have things that store electrical power more effectively than a battery, but you won't like them. Plutonium batteries would easily allow for an electrical helicopter and would allow it to remain flying constantly for 40 years straight (or more if you like, thousands of years wouldn't be out of the question, really). There are a few other nuclear options that would provide similar performance. But I don't think I need to explain why this isn't done. Plutonium batteries have the advantage that they do direct electricity generation, making them very, very small and efficient. Also, in space, temperature is supposedly -271 degrees celcius, but because there's no gas colliding with the craft, heat will take weeks to leak away from the spacecraft. So anything with a heat based generator (which is nearly everything) is out. Which is why plutonium batteries are pretty much the only answer.
[1] http://www.industrytap.com/worlds-first-electric-powered-hel...