60 sounds fair. I honestly didn't see a difference when they changed it to 30, because it takes me 30 seconds to read through all the spam and by that point the person has already posted the exact same message (which pushes up the chat, and when you have 4-5 people who are more or less on the same 30 second interval, you end up being bombarded every 30 seconds while you're trying to sift through potential sales). And it's just a vicious cycle, so I usually keep trade chat off and work for my stuff.

Changing it to 60 would only double the time you get to read stuff, really, as people will still be spamming in intervals.

I'm not sure how difficult it would be to essentially turn the trade channel into a crown sink (programming wise). But I would definitely want it so that you don't get charged for posting once (maybe even up to three times in a row?). And it could scale, as well. So you get three free posts (every hour maybe?), and then you get charged 1k for posts 4-9, 3k for 10-14, 5k for 15-19, and so on and so forth. Then have it reset at some point (like once every hour) so it's not completely debilitating to say things in the trade channel multiple times in a day (ex: You make 10 posts at some point early in the day, then go to make another one 8 hours later and you're charged 3k without realizing, or something to that effect).