Value of "Premium Economy Reward Upgrade"

Hi,
I'm trying to fathom out what the real value of this is.
Since Las vegas is a regular trip for me, I will use it as an example...
A return to Las Vegas in Economy, purchased using miles is 42,500 miles + X taxes.
A return to Las Vegas in Premium Economy, purchased using miles is 70,000 miles + Y taxes.
Clearly, this makes the voucher worth 27,500 miles (unless I am mistaken?), but what about taxes?
Would I end up paying X or Y?
As an aside, am I right in thinking that any reward flight does not yield any miles or tier points?
Thanks
I'm trying to fathom out what the real value of this is.
Spend £10,000 on card purchases on the Virgin Atlantic White American Express® Credit Card, or £5,000 on the Virgin Atlantic Black American Express® Credit Card by the end of any year and you will receive a Premium Economy reward upgrade that, subject to availability, enables you to convert an Economy reward redemption (an Economy flight bought with your Flying Club miles) into a Premium Economy reward redemption on Virgin Atlantic.
Since Las vegas is a regular trip for me, I will use it as an example...
A return to Las Vegas in Economy, purchased using miles is 42,500 miles + X taxes.
A return to Las Vegas in Premium Economy, purchased using miles is 70,000 miles + Y taxes.
Clearly, this makes the voucher worth 27,500 miles (unless I am mistaken?), but what about taxes?
Would I end up paying X or Y?
As an aside, am I right in thinking that any reward flight does not yield any miles or tier points?
Thanks
