Sarr only just turned 21. His 10 goal 10 assist season is quite impressive. He's got great acceleration, and powers past defenders. Coming in off the left flank, he's able to cut in and deliver good shots at goal. Off the right flank he's able to get good crosses in.
He can still improve on his decision making. Sometimes goes into crowds unnecessarily, or sometimes gets the angles wrong on the final ball. But you cannot doubt his positivity and aggression. He has the athleticism of Welbeck with more skill. He won't trick you into submission like Sane but he still has enough to blow past you. With Aubameyang, Nelson, etc, would allow you to have a team that can run at defenders.
He joined for 15m pounds 2 years ago. So 25m pounds would be a good deal, 1.7x money in 2 years is a healthy profit. Even if you push to 2x money that's still very good. He's shown great trajectory since he joined.