Hiring Cost should be a part of the HR Budgets. HR Budgets typically are a percentage of the Turnover of the organization.
You need to allocate the Hiring Cost amongst various methods/ways of hiring. How much you allocate to each one would depend on the Hiring needs and strategies of the organization. For example, if you need to quickly hire your next set of employees, probably you need to allocate more resources to your fastest mode of Recruitment - and if you have the luxury of time then you need to probably expect some more recruitments from other slower modes (which could be cheaper) of recruitments. This is just one yardstick amongst several others that one needs to consider.
There are no absolute numbers or percentages that you could fall back on. This is something that you need to learn on your own according to the needs of your organizations.