The trick is to buy a nice leather sleeve for a standard size of notebook, then buy cheaper inserts.
You might find that you enjoy the quality of paper of other notebooks, but it gets you over that "this notebook is too nice for my random thought"-block.
Doesn't have to be leather - just saying - but also the cheaper notebooks tend to fall apart and I've lost pages this way. One thing about more expensive ones like moleskines is that they are more robust, in my experience. I hardly "splurge" on anything but I realized there is some value to good writing tools.
I instead just got myself a cheap detective-style notepad I can keep in my pocket to write down all the crap the comes to mind, everywhere I am, even while standing up. That's exactly what I need.