Non-response would be more palatable if they at least had an autoresponse so you know your query was received at all.

% of response is just as subjective as the rest of this business. An amazing query might garner a 90% response rate, and a really terrible one might not get any response at all.

When I was querying, I had about a 70% response rate overall. 90% of those responses were "no thanks," but those were better than no response.

The whole industry reminds me of the intro to Whose Line is it Anyway: "the show where everything's made up and the points don't matter."