How much would a shilling or a pound cost in today's US money?

The British pound in no way can be considered equal to the US dollar; they are two different currencies with two different values. So a shilling (no longer used) or a pound's cost in American money is all dependent on the exchange rate which is in constant flux.

http://www.miketodd.net/encyc/dollhist.htm

If this question is about how much you can get for antique coinage:

You should ask your local (US) numismatist about the price he would charge for a shilling coin, or a pound note or coin. The dates are of vital importance of course.

A shilling was 12 pence, but the shilling coin became 5 new pence or p. Pence refers to amounts, pennies to actual coins. A UK 'dollar' was 5 shillings 5/- (a crown if it was one coin), and half a dollar was half a crown 2/6. I think we do it on purpose...

As well as dates being important, condition is too.

Two different answers depending on the intent of your question.

1. If you are asking what a shilling represents fractionally:

A shilling was originally 5% of a pound. As an analogy I suppose you could say that therefore it would be analogous to a nickle (5 cents).

2. If you are asking "how much can I sell a shilling for today?"

As pointed out above, the answer is clearly "it depends".

The value of a shilling is worth whatever market price someone who deals with antique coins is willing to give you.