This was more difficult to figure out than I thought it would've been. Only two of my books covering this period actually mention the exact price on William's head. One specifies it as 25000 gold crowns, the other converting it to about 75000 guilders. This gives us two ways to calculate the value of the reward.

Firstly, assuming a weight of 3.7 grams for a 22 carat gold crown (pulled that number from Wikipedia, I'm not an economic historian sorry), or 3.4 grams of gold per crown we get a total of 85 kilogram of gold. Gold at the moment is worth about 31000 EUR per kg, so the total value would be about 2,600,000 (2.6 million) EUR.

Secondly, using this nifty calculator from the International Institute of Social History we can find out what 75000 guilders in 1580 would be worth these days. The result is that 75000 guilders in the year 1580 has the "purchasing power" of € 1,530,431.73 (1.5 million EUR) in the year 2013.

That's a pretty big difference, which can be explained largely by the rise of the price of gold since the financial crisis. At the start of 2006 the gold price was about 18000 EUR per kg; using that price would give us a result of 1.5 million EUR for 85 kg, whereas the IIHS calculator gives 1.3 million EUR for 2006. I'd call that pretty much even, considering the amount rounding and estimating involved.

So depending on if you go by purchasing power or by pure gold value, the price on William of Orange's head at today's values would have been between 1.5 million EUR (1.9 million USD) and 2.6 million EUR (3.3 million USD). Note that in addition to the monetary reward, Philip II offered rewards of peerage (titles and land) and absolution of crimes.

Sources:

• H. van der Horst, Nederland: de vaderlandse geschiedenis van de prehistorie tot nu (p. 149)
• J.J. Moerman, Geïllustreerde vaderlandse geschiedenis (p. 117)
• Institute of Social History
• Goldprice.org