There are so many ways to personalise your planner too that it's quite easy to spend more on supplies to create individuality than you perhaps initially spent on your planner.
Craft paper, paperclips, washi, cabochons, even glue and laminating pouches can eventually all add up.
There are two things in particular I enjoy making for my planner: dividers and paperclips. It'd probably work out more cost effective to buy them, but then they wouldn't be mine. I buy most of my craft supplies through Amazon, occasionally eBay, and I'm sure if I shopped around I could find them for less elsewhere but I like the convenience. Then there's the satisfaction of being able to say 'I made that'.
Similarly with my dividers, I love the satisfaction of a fresh set in my planner knowing I did that. They may not be perfect, but they're mine.