Your conversions are a bit off.
The old shilling equaled 12 pence. Or, 12 pence made up one shilling.
So, 15 shillings was actually 180 pence. 2 shillings would have been 24 pence.
A guinea was worth 21 shillings, or a little more than a British pound. A pound was 20 shillings.
So if Scrooge gives the maid a guinea on Christmas morning, and that's worth 21 shillings, he gave more to her in one shot than he pays Cratchit for a whole week of work (15 shillings). No wonder she ran out the door overjoyed.
A crown is 5 shillings, so half a crown is 2 1/2 shillings, or 30 pence.
I just found a conversion website online. Who knows if it's accurate, but it claims that 1 British pound in 1843 is the equivalent of $124.43 cents in today's American dollar.

It needs to be considered that those paltry sums, in present day terms, are pocket change, but back in the 1850's the buying power would have been quite considerably greater.
If you recall when Scrooge was eating in the inn and stopped a passing waiter to ask for "More bread", the waiter replied, "That's ha' penny extra sir" (1/2 cent).
That does give some kind of perspective about the value of money at that time, where a penny actually had some buying power, unlike now.
Here in Canada the penny was phased out of our currency a few years ago, so now our smallest coin is the nickel, whose days might indeed be numbered