Forgive the title, I'm not quite sure how to concisely ask the question.
Thanks to several answers on this sub, I've taken a look at the Assizes of Arms, 1181 and 1252. Thing is, I'm not sufficiently educated in currency matters, so my question is - how rich is 15 pounds' worth of land? Would that be an outright knight's fee, or is that merely a rich merchant? Same question for the other wealth brackets specified; I suppose what I'm trying to ask is, who are the people being called out for each bracket? What sort of person in those days would have 'cattle of 40 marks' worth'? (I already know straight-up currency conversions are iffy, and I wouldn't know how to frame that in my understanding anyway.)
Also, not quite a wealth-related question. The text of the Assize of Arms 1252 as I found here specifies for armour 'coat of mail', 'hauberk', and 'doublet'. My present speculation is that the 'doublet' in question is a gambeson, but I'm drawing a blank as regards the coat of mail versus the hauberk, as to my understanding, the two terms are synonymous. Would anyone know what the Assize actually means to say?
Pounds and marks are weight measurements. The original English Ppund referred to exactly 1 pound of fine (later sterling, which is 92,5% silver).
A mark varied in weight, but was usually half a pound in silver.
15 pounds worth of land would be land that produced a surplus of 15 pounds.
Currency conversions are not acually that bad - there are several ways of doing this. The webpage Measuring worth not only has 1270-2012 pound values, but also the best estimates of GDPs, inflation, cost of base goods and labour costs, which allow us several measurements on how much your 15 and 20 (40 marks) pounds are worth.
Using a simple inflationary measurement, those 15 Pounds (I used 1270 as a start, since it is the earliest year I have) are worth 12 150 Pounds 2012. However, this is not always the best measurement, as many goods are much cheaper (relatively) today than they were during medieval times.
However, if you take the amount of work a labourer would have to do to earn 15 pounds then and let him work an equal amount 2012, he gets 221 000 pounds.
If you take those 15 pounds as part of England's GDP then and take the same percentage today, you would get 3 620 000 pounds.
As you can see, it is quite a lot of money - however, I do not think it is out of the range for a well-to-do tenant or yeoman to have cattle of 40 marks if he is a successful man - they would be an investment that would pay off, after all.