According to the chap who runs HawkEye (the technology that is used in cricket and is cited as one of the preferred systems for football) the system would work by basing six cameras in the stands rather than installing them in the goal posts themselves this means that all possible angles can be covered and helps eliminate the risk that the ball may be obscured if only a couple of angles are used.
He reckons you'd be looking at a cost of between
£125,000-£250,000 per ground to install and run the system. Now, he's confident that the cost to install it in the Premier League could be covered by sponsorship but this still leaves the big question of what would happen in divisions below the top flight. Would a sponsor be happy to cover the cost to install the technology at the 72 clubs that make up the Football League, for example? Based on the costs above you'd be looking at a minimum cost of at least £9 million to get it up and running.