How valuable is a bishop?

Could I please get some opinions on whether, in most cases, preventing an opponent's king from castling is worth the sacrifice of a bishop to accomplish it.  Thanks.

Generally speaking, since castling by hand takes 3 tempi, and 3 tempi are about a pawn, a bishop is too much to pay.

In "most" cases, no.

Generally it's a terrible idea, if you don't have a strong enough attack while the king is in the center, but if you are always "sacrificing a bishop just for the sake your opponent can't castle, it's not worth it. The value of castling should not be overestimated in some positions. After all, there is positions I don't need to castle and find my position superior. e.g In the King's gambit-Bishop's gambit, even the move (1.e4 e5 2.f4 exf4 3.Bc4) 3... Qh4+ is dubious to some, even if it prevents white from castling without any cost of material. 

Don't sacrifice a bishop just so your opponent cannot castle, the price is too high. besides, it's still fun pawn storming when even your opponent has castled 

Thanks everyone for setting me straight.  That's definitely one thing I'm going to stop doing!  I also didn't know that a pawn was worth 3 tempi ... thanks for that info, Goldendog.

No. Though the Cochrane Gambit (1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nxf7?!) is somewhat playable. That is, masters have sometimes played it against masters and won. Topalov played it against Kramnik at Linares and drew (if you don't know who those people are, "top 5 player" played it against "one-time world champion" at "really really prestigious tournament").

I'd say the Bishop sac is only worth it if you already have other pieces already positioned to move in immediately.  As white, I'd be glad to lose my f-pawn for a Bishop.  By the time I finish the manual castle, my King will still be protected behind two good pawns and my Rook will be targeting your Kingside down a semi-open f-file.

And, in my latest tournament, a 1590 played Bxf2+?! against me rather early in the game when the situation did not warrant it. It was a rather easy win even though I made a slight miscalculation and had to give him his piece back. If I had made the right moves, he, in fact, would have been the one whose king was in trouble because of the bishop sac and queen foray!

Guys, I am thoroughly convinced ... I'll never do it again!  Thanks to all.

From what I've read on chess.com, a bishop is worth more than a knight, but less than a knight.

How is that possible?

it depends on whether the bishop is good or bad meaning if its being blocked in by its pawns or not. on an open board a bishop is worth half a point more than a knight

A bishop's worth is situational. Typically in closed positions, the knight will be worth more then the bishop, in open ones, the bishop will be worth more then the knight.

sacing a bishop is not worth unless you get positional compensation or material. i dont know what else to tell you. i wish i could elabrate but havent the energy.

One of the reason I stopped playing the Petroff's Defence is because of that gambit...

It really depends on weather or not you have more attack to go with that. If you got like 3 more pieces developed than your opponent and you can get more attack in then ya, but if you are just barely up on development or down / even then no.