The hard part here is figuring out exactly what conditions are being stated. This is my best guess at what is meant.

There are 8 rubies, 10 emeralds, and 100 pearls in the earring.

What is meant by “purchased … at an equal amount” is that the total price of each set of gems (rubies, emeralds, pearls) is the same, although the number of gems included in that total is different.

“Sum of the prices … three les than half a hundred” means that the total price of 1 ruby, 1 emerald, and 1 pearl is 47. (We don’t know the unit of currency, though. Pounds sterling? Rupees? Goats? It’s not said.)

If this interpretation is correct, then whatever the price (call it p) of one pearl, the price of an emerald is 10p and the price of a ruby 12.5p, and therefore
p + 10p + 12.5p = 23.5p = 47
p = 47/23.5 = 2

So a pearl costs 2 of whatever, an emerald costs 20, a ruby costs 25, and the whole earring costs 600.

These numbers are neat enough that I suspect this interpretation is what was intended.