It depends on
what your payload is and what you consider to be cost effective.
Remember that if you intend to rely upon the railgun to supply
all the energy require to boost your payload to orbit, it will have to
accelerate your payload to
better that orbital velocity at luanch: gravity and, more importantly, atmospheric drag, will be working from the
moment of launch to reduce your velocity.
Unfortunately, the G-forces induced by such an intense acceleration would tend to reduce most payloads to worthless mush.
Then there is the cost factor. Railguns use huge amounts of power to energize the magnets of the linear-induction motors most such devices are based
upon. You have to provide that electrical power cheaply enough to make the system cost-effective as compared to more conventional launch systems
(ie.: rockets). And of course, the bigger the payload (the higher its mass) the more power your railgun will require to accelerate it to launch
speed.
Have you priced the construction and maintenence of a dedicated nuclear power-plant recently?
