I've not got one, hard to say on cost, I'd say around £30-60, it depends on the condition of the case, colour and if it has a psu.
I've seen a bit battered ones without PSU go for around $40 in the US but I've also seen them go for around $100
Converting it will require some work.
Best bet, you'll need to cut out the rear section where the motherboard is, get a normal ATX back panel to replace it.
Simplest solution here is to buy a separate removable motherboard tray, remove the dell btx motherboard tray and just put the new one in, add a couple of holes to screw it in.
There's nothing you can do with just "off the shelf" bits it will require some modding but not a great deal.
Here's a log that shows what need to be done
http://forums.anandtech.com/showthread.php?t=2240071
I've found only one for sale in the UK for £160
http://www.ebay.co.uk/itm/DELL-XPS-7...item563bf47ad5
I'm guessing the xps 720 was more common in the US so they cannot price gouge so much.