When I attended the various meetings on the new offices and 'Tesco swap' it was said that the cheapest option was to stay put and redevelop on the existing site, although I recognise that Tesco would be better placed nearer the centre - where it is now going.
It was also said that a purpose built site at Erith would cost (IIRC) £44m (or maybe it was 42) so more expensive than the Woolwich refurbishment at (IIRC) £36m. However the Woolwich building is unsuitable in several ways and most tellingly of all the meeting revealed a 40 year life for the Woolwich and 60 years for a new building - so the current decision might be seen as short sighted.
Much of the Woolwich building was built for document storage, in some cases below ground caverns with no natural light. The building predates modern communications needs and what cabling and trunking there was has been stripped out. A lot of conversion work is necessary to a dilapidated building.
The council has never offered proof that the deal is self-financing, the public was kicked out of the meeting when that may have been discussed but afterwards councillors were complaining they could get no answers either, so the vote for what we have now was blind in many ways. I seem to remember one of your headlines saying as much.
The plan I saw at the time had no space allocated for staff leisure, nowhere for them to eat their lunchtime sandwich inside or outside. The car parking arrangements were also much more limited especially so since council staff are not to be allowed to park free in the Cinema and nearby car parks as they do now. If it was to be free (I am writing from memory) the numbers were to be severely reduced.
If the deal is truly self financing - and we don't know that - then the downside of the deal is that the Woolwich building is badly run down (a surveyor friend took a look at the outside a year ago and wasn't complimentary about its state) and it will never be as good as a purpose design which could have gone to Erith and over the long term the latter may have been cheaper.
The Erith site with the adjacent station has arguably better transport links than the Woolwich site. Councillor Chris Ball would no doubt say much the same as me - not that I am personally recommending a pro-Labour anti-Tory view. God forbid! :-)