A researcher that I know organizes a symposium within a strange conference and has asked me to go. What is the appropriate etiquette for declining?
You basically said it best yourself:


  the funding money for attending conferences is limited and I think I will get a better value for it on another conference.


This seems to me like a perfectly acceptable reason which needs to be enough for any reasonable person. You can't go to every conference where somebody you know asked you to go, and this one isn't high enough on your priorities list.

(personally, I would not lecture the other person about what you perceive as a spam conference that (s)he is involved with - just say friendly and truthfully that this specific conference is not of great interest to you, and that should be it)