Also, isn't a royal dukedom different than say a garden variety dukedom? I would imagine there are many dukes in England at the moment, but not all of them are royal. A royal dukedom (like York, Edinburgh) is not a step down from being a prince, it's a compliment to it.
The only thing that makes a dukedom Royal is the fact that it's held by a member of the royal family who is entitled to HRH.
There are only 8 royal ducal titles at the present time -
the Duke of Lancaster, the Duke of Normandy (both held by the Queen)
the Duke of Cornwall, the Duke of Rothesay (both held by Prince Charles)
the Duke of Edinburgh (held by Prince Philip)
the Duke of York (held by Prince Andrew)
the Duke of Gloucester (held by Prince Richard)
and the Duke of Kent (held by Prince Edward)
and there will be a 9th if Prince William is given one when he marries Catherine Middleton.
Lancaster and Normandy are both tied strictly to the Crown and can only be held by the reigning sovereign. Cornwall and Rothesay are both strictly tied to the heir apparent to the throne, as is the earldom of Chester.
There are currently 24 peers with ducal titles in the UK -
10 dukes in the peerage of England
6 dukes in the peerage of Scotland
2 dukes in the peerage of Great Britain
2 dukes in the peerage of Ireland
4 dukes in the peerage of the United Kingdom
And in medieval times, a dukedom for a prince was considered a step UP, actually. Back then, titles came with landed estates that increased the wealth of the holder and provided income for his heirs.