I just ordered a Vortex Strikefire. They offer the option of a high mount designed for an AR 15 with a front sight base. For other optics you may need to buy a riser. They attach to the flat top rail and then the optic is mounted on top. They come in different heights, so you can choose an absolute co-witness or a lower 1/3 co-witness. The risers also come in different lengths (2 slot, 3 slot or 13 slot).
A riser will run anywhere from $10 to $50 depending on make and model. Replacing the front sight base is not simple, because the front sight base is also the gas block, so if you remove the front sight base you'll need to replace it with a gas block, which means aligning the gas port and detaching and reattaching the gas tube. Besides, if you keep the front sight base you'll still have your iron sights as back up in case the optic fails.