Dont waste any more time on this - it could be worse, at least you got half the value of the account.
I just dont think there is any reasonable way you can get this account back.
The only thing I would suggest is this - if you built the account in the first place then maybe you can use exact same tactics and rebuild a new account in a similar way - i dont really know about instagram but if its anything like twitter then I would build a new account and follow all of the other accounts followers in order to get a lot of them following me back. Use the same type of content to post on the new account and so on.
But aside from that, just learn the lesson that doing (reasonably) big deals like that with people you dont really know does definitely require some sort of escrow service to avoid you getting scammed.