For a crossbody bag for everyday use, I'd pay attention to a few basics: The material should be durable (nylon or polyester last forever), and it really helps if it has separate compartments for cables and small stuff - otherwise you're constantly rummaging through it. A padded laptop compartment is also important so your device doesn't slide around. The size should be fairly compact so it doesn't look like a hiking backpack when you wear it over your shoulder.
Check out the usual suspects on Amazon or in bigger stores - brands like Targus, Everki, or even the in-house bags from MediaMarkt/Saturn are usually solid. You'll definitely find something under 100 euros that does the job. Important: Try it on if possible to see how the bag fits when you pack it with your laptop and your stuff - sometimes an empty bag looks way smaller than it actually is.
My tip: Look at reviews to see if people mention that the straps are comfortable when it's full. That's more annoying than you'd think when you're carrying it around every day.