You don't make a book into a bestseller. Once a book is published, it's out of the hands of the publisher, author (and ghostwriter), and editor. It's now in the hands of the readers. Readers decide what's going to be the next bestseller based on what they buy. A publisher can put tons of money into marketing a book and the book can end up being a big flop. Likewise, a publisher might think a book won't be a big seller and won't put a lot of money behind it and it ends up being a breakout hit. Examples of this is The Time Traveller's Wife and this is the most recent one http://www.nytimes.com/2011/03/12/books/heaven-is-for-real-is-publishing-phenomenon.html?_r=2&partner=rss&emc=rss&utm_source=twitterfeed&utm_medium=twitter
You see, writers make books bestsellers, not authors, ghostwriters, or publishers. So, no one can write you a best selling book. There are many well-written published books that don't get much attention from readers. If publishers and authors knew what would catch on like wildfire then 1) they'd be making more in sales and 2) well I can't say we'd have more bestsellers because there are only a finite number of spots for the TOP, but there would be more competition for those top spots. Instead, there are very few authors that can make the best sellers list and they're usually the names you've already heard of and some new comers from time-to-time.
On to your question, which is equally important and what you really want answered. Good ghostwriters don't come cheap. They don't care how great your idea is because they don't make their money based on someone's "great" ideas. They make their money by writing and getting paid, so you're highly unlikely to find a ghost writer who is willing to take a lower payment just because you can't pay. If you can't pay and you have faith in your ideas, you're better off picking up some books and taking a few creative writing classes to learn how to write them on your own. There's lots of info online and many books and magazines that give newbie writers advice and info about the publishing industry. Even Stephen King wrote a book on writing titled "On Writing.' I suggest you pick it up if you're interested in writing. There are many writer forums you can join and learn from other writers. Otherwise, you're left with saving up and then trying to pay someone.
I know of a few ghostwriters and in a bad economy when they're starving for work they cost at least $7,000. They'll cost more in a good economy. So, as you see, it doesn't come cheap. There's a lot of time and research invested in writing a good book. Once the ghostwriter is finished with it, it's up to you to send it out to agents, unless you make some kind of agreement with the ghostwriter to do it on your behalf. Also, a ghostwritten book does not mean that it's guaranteed to be published. So, you might end up paying thousands of dollars and still not have anyone interested in publishing your book.
If you decide to go the ghostwriter route you'll need to know what to look for in a qualified ghostwriter and that's credentials. What books have they written that has been commercially published? They might not be able to tell you all the titles because of a confidentiality agreement, but there should be some that you can look up and verify. You don't want anyone who has only ghostwritten self-published books. Self-publishing doesn't have any quality standards, so that book could be terribly written for all you know. Here's an example. She only ghostwrites non-fiction books, but if you look at her site you'll see that she has a list of books she's ghostwritten and that you can verify through independent sources. These aren't self-published books and this is how she makes her living. She's also had her own books published, including some books on writing. You're looking for someone who's not trying to "hide" or conceal what books they've ghostwritten and can't verify that they're a good writer; that's a red flag. http://www.jennaglatzer.com/
Alas, you can find some writers message boards and see if anyone in those places have recommendations for are freelance ghostwriters who have a long history of writing commercially publishable books. Many published authors also do some ghostwriting to supplement their income, but alas, it's not cheap. I don't have anyone to recommend because I don't know your salary. I suggest you get recommendations from someone you respect and not search google, because you don't know what kind of filth you'll find there. See if there are any legitimate ghostwriting companies around. Be wary of anyone with no verifiable experience writing and who has never published a book (self-publishing doesn't count). If they've never published a book then how do you know they can write publishable material? You're looking to get published, so you want someone with a proven track record of writing publishable material. Note the word Proven.