Too much information is missing.
1) Travel. From where?
2) How much to park for the time it will take?
3) How much is congestion charge these days?
Take your expected hourly/daily rate, add on the three items above, factor in a fairly big aggravation factor for working in London - while allowing for the windows to be filthy from pollution (maybe need going over twice?), then add on a chunk for it taking longer than expected (fairly normal for me).
If the price seems ridiculous, put it in anyway as most people quote a lot higher for London work. Don't forget time and effort making things safe for pedestrians/traffic etc.
Maybe allow something for the fine when you "accidentally" squirt the traffic warden
.