How much is a ducat compared to in value to real money?
Deliberately indeterminate. Partly because trying to impose an idea of "today's money" on a medieval economy just fails. The value of labour is entirely different, production, supply and demand are different...
The way I mentally have it is; a ducat is enough money to pay 100 spearmen in padded armour, 30 lightly armoured men with maces, 12 men in medium armour with flails or shortswords, or six men in platemail with claymores for a year. It is enough to pay wages for a basic infantry regiment. That is my base medium of exchange and the fiat value of a ducat.
As ever, make an application and I or one of the Kings will accept it if it's not horribly unbalanced.
Edit: But... bearing in mind that the idea of a standing army isn't quite accurate in this setting - regiments typically are somewhere between peasant levies and professional armies right now. They can be considered to be paid a low retainer and then higher amounts in times of active service, but the amount averages out to their wage/year (or it could be considered the value of the land portions/year given to knights in response for their raising levies - it's deliberately vague). If we took the average wage of an urban labourer in 1300 to be our base (~£1.5/yr, perhaps), multiply that by 30 and we get a figure in the region of £45. That's actually pretty damn hefty, considering a mail hauberk fetches something in the region of £5. Then again, that does match up to our ideal of it costing ten times that amount for a small squad of medium armoured men, so maybe it matches after all.
The specific economics don't matter too much as a direct historical map - only the relative economics. If you can pay the wages of thirty urban labourers for a year with it, or retain and periodically hire for up to 30-90 days of service a force of 1 regiment's strength, it's a ducat. But again, these are deliberately nebulous figures.