MPM PE - Upgrade to Upper 1 way

Hi all
So after my recent near miss for access to the LHR CH ii) I have been looking into options to get our 3 x PE seats to LAX upgraded to Upper on the outgoung journey in August.
I called the FC hotline yesterday and was quoted to upgrade our MPM fare (K Class) to UC out, PE return. Quite a lot (3k ish :0 ) so I obviously declined.
I have been looking on a daily basis to see if there are any G's for our dates, as it is a Saturday and bank holiday weekend I am not hopeful.
However I can see 2xGs for the day before ): ):
So my question is this, how much am I likely to be charged to upgrade K to S class fare if possible/available, so that I can possibly grab the 2 Gs.
Do you have to pay extra taxes ?
I have 40k miles in my account which I assume would cover 2 x 1 way upgrades from S to G ?
There are 3 of us travelling so unless another G appears I would just upgrade our sons ticket as above but stick with K class. They can do this for additional £600.
I am probably asking how long is a piece of string but before I hang on the telephone again I thought I would get some input.
Thanks !
L2H

So after my recent near miss for access to the LHR CH ii) I have been looking into options to get our 3 x PE seats to LAX upgraded to Upper on the outgoung journey in August.
I called the FC hotline yesterday and was quoted to upgrade our MPM fare (K Class) to UC out, PE return. Quite a lot (3k ish :0 ) so I obviously declined.
I have been looking on a daily basis to see if there are any G's for our dates, as it is a Saturday and bank holiday weekend I am not hopeful.
However I can see 2xGs for the day before ): ):
So my question is this, how much am I likely to be charged to upgrade K to S class fare if possible/available, so that I can possibly grab the 2 Gs.
Do you have to pay extra taxes ?
I have 40k miles in my account which I assume would cover 2 x 1 way upgrades from S to G ?
There are 3 of us travelling so unless another G appears I would just upgrade our sons ticket as above but stick with K class. They can do this for additional £600.
I am probably asking how long is a piece of string but before I hang on the telephone again I thought I would get some input.
Thanks !
L2H
