If scutage was one or two marks, how much was that worth and how often was that paid a year, I imagine that a professional soldiers are expensive and that doesn’t sound like much money for hiring troops full time

A mark was around 2/3 of £1, or roughly £578.00 in today’s money. So King John’s scutage of 3 marks (or roughly

£2) per knight’s fee in 1214 added up to £1,734 in today’s money. Given that the average yearly income in 1214 was around £2 ½, you can see why it provoked political resistance.

As for how often, scutage was usually levied on a campaign basis as opposed to a yearly basis, so it depended on how many separate campaigns were launched in a given year.