As this is a language site, we may as well start with sorting out the language problems. Try not to get so involved in English literature that you forget how to use the English language :). Here's how you should have asked the questions:
"What was twopence worth in the 1870s?"
"What was the value of twopence in the 1870s?"
and "How much is twopence?"
Bramwell is right that that 'twopence' - or more commonly 'tuppence' - refers to the pre-decimalisation 2d, not 2p.
The best way to work out what it was worth is to compare it to wages. In 1870s London, an unskilled manual labourer earned four or five shillings for working a 10-hour day, which was 50 to 60 pennies. So if you think of the 'twopence' in your story as being maybe one twenty-fifth of a poor person's daily income, you can get some idea of its value. And yes, you probably could get a couple of beers in a working man's alehouse for 2d at that time. It was a not inconsiderable amount for a poor person, but very little for a rich one. Remember also that the gulf between the rich and the poor was much greater in Victorian times.
That's my two penn'orth, anyway. (Sorry, couldn't resist that one.)