Rebuilding using your existing block will save you some cash. You might get a better deal from an assembled shortblock from one of the vendors, but if you end up taking to one of the vendors to let them do the work for you, you'll end up saving some money on the engine itself. But you still have to factor in disassembly of your current motor, balancing of the internals, blueprinting the build, and reinstallation back into the car.
If you're doing heads/cam and other components in the valvetrain (lifters, pushrods, oil pump) it won't really add any to the cost of the installation or assembly of the long block, so might as well just replace the stock, worn out stuff with new, higher performing parts.
Price then hinges on the shop rate and the parts you select.