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? I have been invited to a symposium related with my research field within a conference devoted to a broad area in Mathematics, not really associated with my field. This symposium is organized by a researcher that I met in previous, well-established and respectable conferences. 

The problem is that I have never heard before about that conference to any colleague in my field, if we exclude the emails sent by the conference organizers asking researchers to attend or to organize a symposium within the conference (I have received myself a lot of them in my email spam folder). Indeed, I am really confused about the reputability of the conference itself, and I tried to find it out without too much success in this closed question.

I have decided not to attend the conference given the information that I have at hand, since as a young researcher I want to stay as far as I can from potential predatory conferences. Also the funding money for attending conferences is limited and I think I will get a better value for it on another conference.

Now, my question is regarding the way and etiquette for rejecting the invitation. My initial idea was to be honest and tell the symposium organizer about my reason to not accept the invitation (not feeling confortable in that conference), without making up any excuse such as problem in the dates or whatever. But maybe in that way I will hurt his/her feelings, something that obviously I do not want. 

What will be a honest and polite way of rejecting this invitation?
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)