His transfer fee is not an issue.
Sanchez makes 700k a year at Udinese. I'm sure a contract of 3.5m would get him interested, and if we were willing to offer Aguero 7.5m a year, that's a 4m difference between them, which over 5 years and after tax, would equal 40m euros saved.
Even if we pay 45m for Sanchez we still will have spent less than if we have signed Aguero on a cheaper transfer fee, since Aguero's wages are so much more expensive.
Unfortunately, we have zero chance against Barca. The only advantage we have over them is a guaranteed starting spot.