As said above, the ferry legs are factored into the overall cost of the charter or charter programme. It's likely to be comparitively quite a low value over the course of a lot of flights, or long distance routes.
But obviously, it's always useful to try and use aircraft which are located are close as possible.
Regarding tech aircraft, the answer is subject to the terms of the contract between the parties. (Might be a lease, or just a series of charters etc.) More often than not, it will be the airline's responsibility to provide cover for any tech aircraft. Don't forget that they'll probably need that aircraft for another of their client's in the next few hours or days anyway.