In a practical game the decision which pieces to trade can not be based purely on material value alone. Positional considerations are always of the highest importance.
As for material value, a pawn is usually considered to be worth 1, a knight 3, a bishop 3, a rook 5 and the queen 9 pawn units.
To give you an example of positional considerations over pure material value: A protected passed pawn on the seventh rank is certainly worth more than an isolated double pawn that is blocked by your opponents knight.
If you have the pair of bishops it is often a valuable asset. One of the reasons that is easy to demonstrate is, that in an ending with king and two minor pieces against lone king, it is fairly easy to force checkmate with the pair of bishops, relatively challenging to do so with a knight and a bishop and impossible to do with a pair of knights.
Trading two minor pieces for a rook and a pawn is mostly considered equal value in material, eventhough it creates an imbalance.
Please note, that some pieces work together especially well. Like a queen and a knight for example. They are often the better combination than queen and bishop.
It is less important which pieces go off the board, but more important what remains on the board.
Queen is worth more than a rook