How much were the 30 pieces of silver (Judas's payment) worth?

Assuming that the 30 pieces of silver are the tetradachmas that Thayer suggests, the total weight of the 30 pieces of silver would be 15 troy ounces, or something not too far from 1 pound of english measure. (Not that most Europeans would need the equivalent, but its roughly 466 grams). Per this source, 1 troy ounce of silver, trading on May 20, 2013, is $21.64. The total dollar value of 15 such ounces would thus be $324.60. Phrased exactly according to the terms of your question (What would the value of the metal be), that is exactly your answer.

That said, it's also a completely bogus number. You really are much better off thinking of it as half a year's wages. Here's why:

1. $324.60 would have an impact far in excess of the current value nearly 2000 years ago. The things that one could do with $324.60 in today's terms is completely incomparable to what could be done back then. To say you could maybe a small computer with that, for example, is completely irrelavant. Even to compare that "two weeks worth of groceries for my family" is bogus, because $324.60 would fed a family for a heck of a lot longer than that.

2. The price of the metal discounts all sorts of other economic factors. Today, silver is a rather nontraditional investment. Back then, it was money. We no longer peg the dollar to precious metal. They did.

3. Pieces of silver were regularly adulterated - once monarchs learned they could shave the value of the metal in a coin, inflation became commonplace. Even if we knew what type of coin, we don't have data as to the purity of the coin in question.

4. The concept of a middle class was much more of a rarity. 30 pieces of silver to the very rich was very little. To the very poor, it was a lot. Most moderns in the West are neither, and as such the emotional reaction to the money (which after all, really was the point for Judas) would be very different from the actual value.

5. Should you be able to unearth these coins, their historical value would well outstrip the actual value of the bullion. I know where I could put my hands on a parchment that was signed in 1776. I guarantee you that its best sale price would not be fetched by carefully removing the ink where a bunch of American white guys put their 'John Hanc**k''s on it and reuse it as a sheet of paper.

6. The time element really is significant. The minimum wage in the United States is approximately $7.25/hr. That means, in the United States, it would take a maximum of 45 hours to earn that much money. As a country, the United States is sufficient rich in labor saving devices and capital to make that a reality. In Bangladesh, the minimum wage is $0.11/hr, meaning that it is not unreasonable to assume it would take an individual 3000 hours (or nearly one and a half years working 40 hour weeks) to collect the same. The difference between 1 standard week and more than 17 months is due to a difference in living conditions between two countries at the same time in history. When you start doing temporal calculations, the math gets way worse.

As a result of each of these concerns, plus others not listed, Wikipedia is right to list in terms of days' wages. Imagine, for example, what half a year's wages would mean to you (regardless of your socioeconomic status). That struggle - choosing between money and God - was what was going on in Judas' heart. (Well, amongst other things, at least). To compare apples to apples, you really should be comparing your situation to his. The best proxy I can think up is a percentage of your regular income. Half a year's worth of my salary would certainly be tempting indeed.

Note: More on the drachma can be had at this link