The trouble with conversions to real money is that it all depends on the era and setting.
For example, converting at the time of the Domesday Book would be different to the time of the Wars of the Roses, simply due to inflation and currency changes.
I only have RQI Empires, but looking at that, if I read it correctly, the WLTH of a state represents the annual income of that state. If you can work out what the annual income actually is then you can work out approximately how much 1 WLTH represents.
For example, Medieval England at the time of the Angevins had an income of 36,000 pounds, or 8,640,000 pennies. Assuming a WLTH of 12, this gives 1 WLTH = 720,000d or 3,000 pounds.
How to translate that into local economies? Work out the stats of the major dukedoms, then work out the stats of the minor holdings. Once you get down to the land that the PCs hold, you will have an idea of the WLTH of those holdings. At that scale, you probably want to multiply things by 10, or even 100, to avoid fractions of WLTH. Do a calculation of the cost of running the holdings and the amount of WLTH left over. Once you have that, then factor in the cost of building and maintaining a castle and you can work out how much WLTH is needed to bolster the PCs' finances.
It's all a bit wishy-washy, but the important thing is to work out the finances of the kingdom, then drill down to each level.