the A version is the new one, most likely they consider it a replacement for the original. it probably doesn't cost them any more to produce the A version however. so, they probably produce far more A versions than the originals now (if the originals are even still in production...) so they make the original more expensive so people will buy the A version; eventually this will "phase out" the original version. companies using this part wouldn't appreciate suddenly being FORCED to switch to the A version, if the original was no longer produced, but microchip is carefully 'urging' people to switch to the A version by messing with the prices. eventually, nobody will want to buy the original 877's at all, and they can stop production without getting yelled at by various customers.
When a manufacturer stops production of a part, they have to keep a 5-year supply of them (or something like that) on hand. they make this easier by raising the price a lot; eventually nobody will want to buy them, so they don't need to keep a large supply on hand.
Just think: what if the original was cheaper? if some major manufacturer came along and wanted to buy a huge number of the original chips, and didn't want the A versions because they were more expensive, then they would have to keep producing BOTH versions in large quantities. it's not cost effective to be producing both versions, when they could just as easily only make the A version and not lose any money on it.
if only they had put in all the features they wanted in the original version then this wouldn't be a problem.