The high points of prosecuting a patent:
You can do a basic search just using Google (that's where I start); then read everything (patents or otherwise) that it hits. After that, you'll need to find a way to search the patent databases. You can start with the USPTO (for US) and
WIPO (for (limited) international). If ANYTHING turns up that's your idea - you're dead in the water. (All of this is FREE. If you can turn something up and end your search, you've saved at least a few hundred, if not thousands of dollars).
You said you hired a firm to do the search, which is a good idea if you've never done any kind of patent (or other literature search); they will have access to global patent databases and the expertise and experience to find prior art. If nothing turns up to block you, then...
Then you have to write your application - I strongly suggest you hire an attorney to do it. There's a specific way to compose it, and there's an art to including the moon and stars to broaden your coverage. If you've never done it, you'll leave a lot of loopholes open. This step will probably be your biggest outlay of cash (I'm going to guess a good round number would be ~$10K - could be more, could be less depending on how complicated your idea is/how crowded the field is). You will need to pay for the lawyer time to educate him on your idea, him to do the search (already paid for if you use the same firm), and then to write it up. Then you'll go back and forth making sure he got your (main) idea covered.
Now you have to file the application. A good first step is to file the US Provisional Application. It's not that expensive, and it gives you a year to file the (final) application. You can then use this time to develop your idea further, and to shop it around to see if there is a buyer. You can add new embodiments and further examples during this one year time to bolster your case/broaden your patent. (If you do talk to people about it after you file (and don't talk to anyone before you do), be sure to have them sign an NDA (non-disclosure agreement) that prevents them from filing their own patent. You will want to make sure that THEY do not discuss how to improve your idea - because then they become co-inventors.
Since you filed in the US, you will clearly be getting US coverage. The next consideration you will need to think about is global coverage. If you don't want it, just stick with the US. If you do, then things get a bit more complicated (and expensive). No one gets truly global coverage; you will want to get patents in key countries to essentially give you global coverage. What countries you go for depend on the idea.
Since you filed the provisional US application, your priority date is the date of that filing (if you skipped the provisional and went directly to an application that's your priority date). Some countries require that you file directly in their country, so that means you have to file on the date you file in the US (final application).
Most countries, however, belong to the PCT (Patent Cooperation Treaty) which allows you to file a PCT application (these are the WIPO above) within a year of your priority date (if you did the provisional route, this is usually filed at the same time you filed the final US application). The PCT will assign an "examiner" who will do a search and render an opinion on the patentablity of your application; this is not final, nor appealable. It's just a first pass that other jurisdictions may/may not take into consideration.
30 months after your priority date you will then have to file in individual PCT countries. If that country's official language isn't English, you'll have to pay to have it translated - Chinese will run you about $8-10K; Spanish about $3-4K. And you will likely need representation in those countries - your US attorney will set that up. So the PCT buys you 30 months (to not pay translation and filing fees) and gives you a first pass patentability opinon. It's not a bad deal.
Your patent will publish in the US (and through WIPO) 18 months from the priority date (or 6 mo after filing the final application if you did the provisional and filed the final a year later).
During this time you will likely get a rejection from the US PTO - no one ever gets a patent on the first draft. Then you will have to respond and go back and forth with the examiner to get your claims finalized. That can be quick or it can take months/years.
Same will happen with all the individual countries you've filed in. As you can see, a single application can balloon quickly into many applications around the world.
Once you get a patent granted (in any country) you'll have to pay a patenting fee, and then maintenance fees every few years to keep it active. Patents have a lifetime of 20 years from date of filing (in most countries); the fees increase as the patent gets older.
Overall, a good rule of thumb for "global" coverage over the life of a patent is ~$250-300K. US alone is probably ~1/10 that.
That's the highlights. There are many nuances that are particular to the field you are in, the country(ies) you are going for, etc.
