But speed is a ranking factor, albeit small -- like every ranking factor. And Core Web Vitals ranking factors are coming too. If you are monetizing your site at all, $50 one time seems a small price to pay to improve your site speed ongoing.
Google's John Mueller explained how Google's Mobile First Index will index and score sites that have Desktop, Mobile and AMP versions
www.searchenginejournal.com
Google has said they will use AMP for speed testing, and non-amp for indexing.
The other factor though how much better/worse is for AMP vs your standard Xenforo pages:
@mazzly - do you have a
http://developers.google.com/speed/pagespeed/insights/ comparison between the AMP page and the non-AMP Xenforo page?
The other issue though with AMP is monetization is not as straightforward. You can't use traditional banner tags. Google Adsense has an AMP solution, but you should check with your ad provider if they support AMP or not. And it monetizes worse than non-AMP traditionally.