What is 30 pieces of silver in the 21st century?

That depends on what you mean by the question! It is assumed that the piece of silver referred to was a Shekel, which weighed about 15 grammes (about half an ounce). As a commodity, silver is worth around $20 an ounce, so in raw terms a Shekel is now worth $10, so 30 Shekels is worth $300 (£210). However I guess what you are asking is how much it is worth, adjusted for inflation. Well one Shekel would have been around a week's wages for a farm labourer which in today's money is about £290 ($410) in the UK, so 30 Shekels would be about £9000 ($13000). We know Judas went off with his 30 pieces of silver and bought a field, so that $13000 estimate is probably realistic.