This, like most hobbies, is as expensive or as inexpensive as you want to, or can make it.
If I wanted too (or had resources to enable me with the expensive option), I can go out tomorrow and spend $500 and get everything for a GOOD park flyer that will last me, OR I could spend $15-25K on something like a CARF P47 with a 250 Moki .... and then go and buy another one of their different planes each year.
So the amount you spend is entirely up to you, and there is no right or wrong answer and/or amount. I know a guy at my club who gets as much of a kick from flying their (modded) parkzone P51 as they do flying their H9 60 sized Spitfire - which is over 4x the cost. Jury is out on his H9 B25 though.
So, as far as this being an expensive hobby in comparison to others, I sold 'kite buggy' gear that cost me over $5k before I brought my nitro heli (first big ic model) and to date I don't have $5k invested into this hobby (not including the cars and yachts I have before getting airborne either) ..... but if I had unlimited resources, the sky would be the limit on what I could spend on this hobby as that aforementioned Moki powered CARF P47 would be just the start!