there are the on-line offers, from BA.com through MMB and those at check in desk,
The check in prices are standard
WT to WT+ £179 going US East coast and £300 (or £360 I am not very sure from WT+ to CW), of course you should have availability. I am not aware of other prices but everyone can ask at the airport.
The on-line prices can be different from flight to flight, for example if you are traveling in CW and it is already overbooked then BA will offer through MMB very low prices to F. For example a friend of mine had an offer of £150 from WT to WT+ on the route LHR-EZE.
Keep in mind that the on-line upgrades offers are not available for flights exUK domestic as there is a glitch with BA’s system :s
If you originate your trip ex-UK domestic you can get an upgrade air-side at T5 at the BA desks with the signs (BA upgrade offers or something similar)
Also the price for a WT to WT+ upgrade (Aat the airport) on the route LGW-GND costs £179
All those upgrades take the miles/TPs for the higher class you traveled, so yes if you got an upgrade for £100 from WT to WT+ you will get the extra TPs but most of the times you will have to call as ba.com has problems.
TIP: If you are on your way back to UK those prices are significantly lower as £60 represents the additional APD.