51% Attack can be done technically. But in practice it is very difficult to have such a thing. Besides, instead of budgeting, other ways can be tried. If you are considering manipulation for a coin, all you have to do is simple. Don't make purchase after sale. You direct the price that simple. This is not ethical behavior.
I do not propose to engage in financial manipulations, purchases and sales. The need to buy a large amount of coins for a successful attack by an attacker is an obstacle, because Its stupid to attack the blockchain in which he invested a lot of money. Losses will exceed gains. As long as an attacker controls less than half of all coins mined, it will be more difficult for him to carry out an attack than in a blockchain with a standard POW algorithm.