Since 16 June 2014 there is a letter in unicode: \u20bd representing RUB currency sign:
₽
So, the answer to the question: HTML entity which should be used for RUB sign is ₽
Though, I suppose currently not all fonts are capable of showing it. Ability to see the sign in browser generally depends on your system fonts. As of today my Mac OS web fonts are showing it correctly, but I didn’t check it on Windows and Linux.
Therefore it is not recommended to use this new sign when it is critical for reader to see it relying only on system fonts, that might be stale. If you want to be confident that the sign will be displayed you will need to provide your own font at least for paragraphs that may contain RUB sign.
From Wikipedia:
On 4 February 2014, the Unicode Technical Committee during its 138th
meeting in San Jose accepted U+20BD ₽ ruble sign symbol for the
Unicode version 7.0, the symbol was then included into Unicode 7.0
released on 16 June 2014.http://en.wikipedia.org/wiki/Russian_ruble#Currency_symbol