One thing to keep in mind, for all the bitter rival fans saying "Oh Sane's injured for two months so Pep splashes £60m on Mahrez," they're missing a big point.
It was abundantly clear we wanted Sanchez, either in January or the summer; however, we weren't willing to compete with what United offered him. We were/are clearly looking for another forward.
We've also known for a couple weeks now that Pep and Txiki have been looking at wrapping up most of our summer business this month, with the World Cup looming as well as prices going way up and a shortened transfer window.
If we wrap up this Mahrez deal before the end of the window, that just leaves Fred for the summer. They can moan all they want now about what we're paying but if we bring in only Fred (and some cheap CFG projects) in the summer, we will almost certainly make a profit on player incomings/outgoings for the next window.
I also like the fact Mahrez is almost 27. At some point you can only buy so much young talent. Having a player at the prime of his career to compete with Sane and Sterling will up their games even more. Added to that, he doesn't block their spot as much long-term.