A regular London cab (the black ones you hail on the street or call by phone) will charge about 50-80 pounds. For a better deal, it’s often possible to negotiate a rate with the driver of a metered cab, especially if you arrange it the day before. Expect to pay about 25-35 pounds in this case.

‘Minicabs’, which are normally unlicensed, are much cheaper, maybe about 20-30 pounds.

Your best bet is to get a regular cab to Paddington Station, and take the Heathrow Express train. It’s much faster than taking a cab all the way, and the train only costs about 11 pounds.

Definitely do not get a bus from Charing Cross!
Will take bloody ages!

A taxi from Charing Cross is around £70-90 on the meter ( i got one back to the office near there a few months ago)
Its charged by the ride and not per person or by luggage.
Basically you fit 5 people in a cab with some luggage so less people gives you more room for luggage!
Definitely go with the advice to book ahead if possible if you want a cab all the way, or arrange a minicab (private hire comapany not licensed to pick up on the street).

I woudl agree with Ferg2k… get a black cab from the front of Charing Cross station to Paddington (probably around £12-13) and then get the Heathrow Express (which is about £15 per person). IF you didn’t have luggage I would recommend the tube. It takes a long while (about an hour) but you dont have to change (unless you want Terminal 4 where the tube station is closed at the moment).