500 Rupiah Makes No Sense

Does anyone know why they made Indonesia 500 Rupiah coins feel like plastic toys?

Because aluminium is cheap and that is also how aluminium feels?

Yeah, if you have ever had another aluminum coins like 1 yen that can float on water. Aluminum do feel like toy coins. 

But the serious answer is modern coinage are minted for its durability to be used in circulation (to decrease usage of paper currency because they tend to disintegrate rather quickly in circulation) for as long as it can be and it just so happens that aluminum perfectly does just that. Durable and cheap. 

They only have 4 current coins, a 100, 200 and 500 rupiah in aluminium and a 1,000 rupiah in stainless steel

I have noticed the coins are similar size and there are 2 series, this is the earlier one with nature. The later 2010s series shows famous people. The earlier 1000 rupiah stainless steel showed a bridge I think (Its in my world collection). There is also a 1990s 50 rupiah coin in brass, likely obselete now.

The rupiah is very worthless around 10,000 to a New Zealand dollar, meaning 500 rupiah is worth around 5 cents, no wonder its aluminium. Their smallest note is 1,000 rupiah which is like 10 cents, their biggest is only 100,000.