There is a well known article called The Evaluation of Material Imbalances
by GM Larry Kaufman, first published in 1999 in Chess Life, winner of the 'Best Instruction' award from the Chess Journalists of America, that has the bishop pair valued at half a pawn.
A quote from the article (linked above):
"The bishop pair has an average value of half a pawn (more when the opponent has no minor pieces to exchange for one of the bishops), enough to regard it as part of the material evaluation of the position, and enough to overwhelm most positional considerations. Moreover, this substantial bishop pair value holds up in all situations tested, regardless of what else is on the board. This large a bishop pair value is surprising because in the opening grandmasters will often give up the bishop pair to double the opponent's pawns or to gain a mild lead in development, factors which are generally not worth half a pawn..."
That same article values the other pieces like so:
King = 4 (endgame)
Queen = 9.75
Rook = 5
Bishop = 3.25
Knight = 3.25
Pawn = 1
Whether you wish to rely on such figures is up to you. Personally I think you would be better off exploring why the bishop pair is considered an advantage, and when. The best explanation I've seen is in Ludek Pachman's Complete Chess Strategy Vol 1.. Some of the points he raises:
- "In open positions their effectiveness is an extremely important factor."
- "...the opportunity of purposeful simplification."
- "The side with the two bishops can bring his king to the centre more easily under their protection"
- "...helps by preventing the enemy king from using the otherwise weak squares of a certain colour."