It all depends. It can cost anywhere from $10 (clomid, without monitoring) to more than $10,000 (IVF with injectible meds). The chance of multiples are not increase if you're only doing natural cycles with an IUI for example. If you add clomid or femara, the risk is slightly higher. And with injectibles, chances are higher than that again. These are all based on doing IUIs or just timed intercourse