The cost of jacking up floor joists may not be your biggest problem. I would be more concerned as to why they are sagging. Joists not sized properly? Posts sinking do to much water under the house or the foundation was built on fill? Foundation problems elsewhere? Beams wrongly sized or improperly placed? Broken joists or beams? If your lucky a 20 ton bottle jack and some shims will solve the problem. Take a good look before you leap.