And to be a bit more precise.

A return First RFS to Moscow is £55 plus 60k Avios. You can do it for a bit less, if you return to the UK in CW, WTP or WT.

The Russian visa, if you sort it out yourself with the consulate's handlers, VFS, will be about £85 for UK citizens. You will need an invitation letter too, which the more upscale hotels will provide for free so long as you pay to stay in their establishment.