Outwell - Classic - Sleeping mat tested

What is the difference between the low range price self inflating mattresses and the wildly expensive ones? We go camping a couple of times in the summer in Ireland so I know I need a three season only as it won’t get below 10C at night.
I used to use a single futon but my hips are sore on that now. So I need a new solution and some food advice please. It’s tricky because ideally I’d like to lie on it before I buy it but that’s not possible. I do love your companies ethos and commitment to ethical and sustainable products so please bear that in mind when you reply. Budget is about E150 and I’d like a wide single/small double size. My tent is a 5m belle tent so weight and packing size are not a huge concern. Thanks so much