For instance, if we know that in 1250 a master mason was paid 1 sterling a day, or that a building project cost £15,000 to complete is it possible to translate that number into a modern day equivalent? I've seen scholars try this, for instance A.J. Taylor in his works on the Welsh castles of Edward I suggested that £100,000 was the equivalent of £10,000,000 in the 1950s. If you can calculate it, how would you go about doing it?
You could conceivably come up with some sort of a number. That said, as a medieval historian who regularly deals with specific amounts of money, it's far more worthwhile to translate a wage in terms of something tangible like a loaf of bread.
The big problem is that no economy in the Middle Ages was entirely cash based and by converting a number into modern currency imposes modern anachronisms on premodern societies.
A better way to think about how much a castle would have cost would be to measure what percentage of the total crown revenues in relation to other expenditures. For the master mason, it would be better to think about 1 pound sterling in terms of rent, food, even a dowry since that would have been a major economic moment in one's life etc.
AJ Taylor was part of an earlier breed of historians when that sort of conversion was more widely accepted.
Sorry to disappoint.