I described the process of raising medieval armies, and paying scutage HERE

The section of my answer that directly handles this query:

Alternatively, if you could not provide the requisite number of men, either because you needed them at home, or did not have enough able-bodied men who could be equipped in time, you could pay scutage instead. In the second half of the 12th Century, the going rate was £ 1 for the 40 day term of service. In Richard I's reign, this was characterized more as a fine that a lord paid to not participate in a campaign. The King could then use this revenue to pay for mercenaries, equipment, supplies, etc. The King might also grant his vassals permission to use scutage among their own underlings, with the caveat that the tenant-in-chief was still required to produce the requisite number of men.