It's actually quite difficult to compare the performance of Chelsea players because they have a lot of competitors there so when Arsenal are interested in Noni Madueke it means they are looking at it from another side. Apart from that, Madueke is a young player 23 years old well maybe Arsenal wants to implement long term investment, who knows in the next 1-2 seasons his market value will reach 100 million.
In the long term, it might be a good idea for Arsenal to bring in Noni Madueke because he's still quite young, and if his performance continues to improve, it's not impossible that his contract value could reach that figure in the next season or two.
Arsenal also has options when the manager is trying to rest players, as Noni Madueke's presence could be a viable option.
Arteta certainly has a vision when recruiting players, although most people believe Arsenal need a pure striker, Noni Madueke's arrival is expected to add depth to the squad making Arsenal even better.