The publisher may not actually have copyright to the song you're trying to arrange. They may only have copyright to THEIR arrangement. You should contact the actual rights holder, which is what Tresona does for you. You could also try to get in touch with the actual songwriter if you want, but good luck with that.
The cost isn't just from Tresona - it's from rightsholders attempting to monetize their works. People have been doing things for free for decades, and now songwriters want to get paid for their work. It's not out of line to expect a thousand dollars for someone to use your hundred or so hours worth of effort and songwriting. Frankly, I'm surprised it's so cheap.