IMO, if its the right player than the cost doesn't really matter so long as you can afford it. Obviously, you still want to get the best possible deal but the number one priority should be getting the best possible team together.
Another thing I have said in the past is regarding free transfers. If I wouldn't be prepared to pay for a player, why would I want him for free? Money is money, just because someone is cheap, young, full of potential, proven in this league, etc does not mean they will make for a good signing.
Berahino may well turn out to be a quality player but he's not going to change our season around is he? Right now, we need quality players that would make a immediate impact. We have plenty of youngsters that could well see us good for the next five years or more already.