There is no such thing as an international patent, so it is necessary to file national patent applications in each country in which you eventually want a patent. It is possible to delay the date on which you have to file those national patent applications by using the PCT system.
A patent application filed under the Patent Cooperation Treaty PCT basically acts as an international placeholder. A PCT application can claim priority to an earlier-filed national application. National applications must be filed within 30 or 31 months (depending on the country) of the PCT application’s priority date to get the benefit of the priority date.
As for cost, it depends entirely on how many and which countries you want a patent in. I usually tell clients to budget somewhere in the ballpark of $50,000 (US) for a set of national filings. Sometimes it can be much lower (e.g., $10K to $30K), depending on which countries you file in. The largest part of the cost tends to be the translations.
But that’s just the start, since most countries will also conduct prosecution of your application, so over time you will likely see prosecution fees of ~$10,000 to $20,000 per country, plus each country will also charge “annuity fees” (also called “maintenance fees.”), which will run another few thousand dollars per year (annuities get increasingly more expensive over time, since most countries use a rising fee schedule).
So, it’s not cheap. You’ll want to be earning revenue from your invention (or at least well on your way there) once you start spending money on national patents. As mentioned above, the PCT system is a great mechanism to delay those costs for a little while.
There’s some more info in my answer to a related question here: