Are you getting temp contractors or permanent employees?
If you need that nginx module built and can't figure out how, get that guy whose done a dozen of them before for a month.
If you want someone to add to the team, looking for very specific skills is going to cause you to repeat this process so in three months no one is around who can optimize an SQL query and you need a specialist for that.
Hire both. Assign the permanent employee to watch the temp contractor and learn how he or she does the work. Then assign the temp employee to watch the permanent employee to make sure he or she does the work correctly.
Of course, that only works if your permanent employee is not willing to jump ship to be the temp contractor with specialist skills and specific experience somewhere else.
There must be some valid reason why businesses would pay more for recruiting than for training and retention, but I am not familiar with it.
If you need that nginx module built and can't figure out how, get that guy whose done a dozen of them before for a month.
If you want someone to add to the team, looking for very specific skills is going to cause you to repeat this process so in three months no one is around who can optimize an SQL query and you need a specialist for that.