Yes, that is correct. You could also say, "If the football club is that rich, imagine how much it spends on players." Or, "If the football club is that rich, imagine how much it must spend on players." The differences in meaning between the three are very small.

It really depends on the situation and if the club is actually going to spend the money. If you have a team like Manchester City who are rich and spend that money, you would say: "If the football club is that rich, imagine, how much it will spend on players". However, if the club has money, but will not spend it. For example, a few years ago Arsene Wenger at Arsenal was criticized for having money but not spending it. In that scenario you would say, "If the football club is that rich, imagine, how much it could spend on players"