50m euros would be massive now or in 2 years. Football spending does not grow consistently. It spikes up due to significant interruptions to the market - e.g., TV deals or the entry of moguls. I do not foresee significant shocks in the next two years, so 50m for Mahrez will still be massive then.
But then you look at the alternative: spend nothing and see more of Walcott looking about as coordinated as a newborn impala, while Ox watches from the stands on medical leave. No-brainer!