it varies massivly,
depends on the quality of the convertion, (mounts welded properly, wiring properly done etc)
the condition of the car, (no rust or dents)
the condition of the engine, (not ragged, well serviced, etc)
the safety aspect (has the suspention and brakes been done? is the chassis reenforced? etc)
so basically there is no way of telling without looking at the car