How much is the right to castle worth?

Say you're in a situation where you can either lose a pawn or lose your right to castle. It's obviously very important to generalise here but let's say the hypothetical situation is maybe 10 moves in with 6 points worth of material exchanged. What would you choose? Is it worth less than a pawn? Half a pawn, quarter of a pawn? How can you judge when it's important?

The only way to permanently lose the right to castle would be having to capture with the king. If you are recapturing something that took a pawn the rook can now get past the king anyway and so you easily castle by hand.

If it was a trade of queens (so your king had to capture the enemy queen) then you generally aren't concerned since the queens are gone.

It all really depends on the situation on the board though. There are some times where a sacrificed knight to ruin the opponents castling leads to a big attack on the king.

Sometimes it is better not to castle and let the king roll up its sleeves. Such a situation is more likely when a lot of pieces have been exchange, especially the queen. The king might also be safer in the (blocked) center than on an open wing. In other circumstances your king's life depends on castling. It is hard to describe the value with just a number, but I would think that in general castling is not worth a pawn. May be two tempi?

In my very limited experience, if a king is denied the ability to castle in a single move, his defenses are usually being stripped away so quickly that he's already in a fight for his life.  At my low level of play, I would give up 2 pawns for the right to castle or to deny it to my opponent.

I guess, being denied the right to castle usually means I'm having other problems with the game anyway.

It's hard to say without a concrete situation.  I like to keep my pawns, because I'm always thinking toward the endgame.  If the center's locked, and a few pieces have been traded, and it won't mess up your piece coordination, these are all things that can make castling unnecessary.  In most situations without any immediate tactics, I'd say keep the pawn.

its usually worth a pawn

Interesting idea but the whole concept of giving values to pieces and pawns is flawed since the actual value of a piece depends entirely on its position. For example a Knight can deliver smothered mate while a Queen cannot.  So which would you rather have in such a position? Castling is a valuable resource but in many positions castling is not required. On the other hand if you need to castle to save your King, it would be well worth a Pawn sacrifice. 

In your situation it is worth a pawn. If the queens have been exchanged, no.