Costs? LOL!!!! Your better off running glow in there. 1st, this plane is probably gonna be a beast. Well over 7 to 8 pounds I imagine.
Your looking at bare, and I mean bare minimum 200 to 250 dollar brushless. Now comes some more $$$....ESC....Probably 70 amps or so. Thats almost another 200. So right now, you got 450 bux into this. I can get an Evo .61 two stroke at my LHS for $99.
Also, you will have to reconfig that batt pack of yours into like 18 cells. That may be too much weight so you will have to go lipoly. And guess what? Another big dollar item!!! I'd say around 200 to 300 for just one batt pack.
Electric powering planes is very doable and less cost to an extent. Once you get to a certain size, your better staying glow unless your related to Bill Gates. Take that money your gonna spend and get a Saito or OS 4 stroke.

Dave...