It's working I have tested, actually it will work for all the new requests but for all those request which are already cashed by browsers need to be refreshed manually or let the browser decide. But once it will refresh it won't be cashed any more.
It should have worked can you please try this, just make sure you are changing in the correct .htaccess file related to same website
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [R,L]
Edit:
Opps sorry to tell you, yes your price URL is perfect, but unfortunately it's not going to work here on BitcoinTalk reason is because browser doesn't see the same URL . Your price URL will be replaced by bitcointalk image proxy. It will be something like
src="https://ip.bitcointalk.org/?u=https%3A%2F%2Fblackhatcoiner.com%2Fprice%2F&t=630&c=_BnnItB_xeGbWA"
And of course browser will cache this, so there is no way you can control this and yes my first statement was correct you can blame theymos for this (again that's not a bug or issue it is suppose to be like this).
If you test your URL directly in the browser you will see it works fine and refreshes price every time https://blackhatcoiner.com/price