Honesty, I do think it's alright to be charged to mine a spawner but I do agree that 15k is too much just to mine 1 spawner. I also think that depending on what spawner it is or how much it costs / you can make off of it, it would cost less or more. Maybe something like 1k or 5k would seem more appropriate for a spawner like chicken but make 10k or 15k for something like a cow spawner. Although I am not 100% sure why a cost exists in the first place, here is my 2 theories. So please, hear me out.
Theory #1.
So you're on your island and you add some new people right? Now you can't be sure they won't steal but you give them that chance. So a person on your island gets out their silk touch pickaxe and goes to mine a spawner. Now that warning that it costs money pops up and they can't afford it. This could stop people from doing that.
Theory #2.
So another theory could be that people can make money so quickly with the spawners that a small charge would be nothing compared to what we make. For example, you can make a few 100k from afking your cow spawners in a certain time frame and if you wanted to move it, you'd pay the small fee which could just be a small 'cut' of what you make in the run of a day from afking that spawner.