How to I get rid of my helicopter on GTA V?
I am sorry to tell you that there is no way of refunding in-game money. Here is a link to someone wanting to refund himself, yet to no avail (Rockstar Forums)
PS.: People seem to be rather rude when asking for this on the official forum, so I'd advise against it.
As @Crowley Astray points out, it's not possible to refund money spent in game.
Unfortunately, you can't even sell Pegasus vehicles, so you can't even make back some of your money.