So I'm not being original here, but since I typed all this out today in reply to someone with an idea but no writing experience I thought I'd make double use of it. Today's inquiry did add "ha ha" after the suggestion that perhaps I write the book and they simply collect royalties for supplying the idea - but I've certainly had straight-faced proposals for going 50/50 before. The other party would give me the idea, I would write the book, they would collect half of any profit.
The difficult thing about writing a book is writing it. Using the language in a way that makes the images and emotions in your head appear in someone else's. Sentence by sentence it's difficult and requires a natural talent honed by a *lot* of practice. Chapter by chapter it's difficult, requiring the plot and pacing to keep the reader with you. You have to grow people on the page, people who are not only realistic enough to suspend belief but fascinating enough to stop the reader wandering off to watch sport, play video games, put on a movie, or spend the day Facebooking or watching cats on youtube.
None of that's easy. When someone comes to a writer with an idea, offering to go halvsies or thinking they've broken the back of the job and now just need to finish off by getting the words down (as people often do) it's a bit like going up to a sculptor and saying 'I've got an idea for a statue, it's an angel spreading its wings, but the clever bit..' and assuming that the job of cutting the form from the stone and making the description into something concrete is just a formality (or at most, half the effort). It's almost like going to NASA with a suggestion of which planet / star / asteroid they should visit and considering that to be a substantial contribution to the resulting voyage. Ideas are like that, they're like destinations. There's a galaxy of them twinkling in the darkness. Pointing at one is easy. Building the rocket and getting there in one piece is hard.
The only advice I have - and it's rarely welcome because it involves a huge amount of hard work and guarantees no success - is that if you've never written before, you need to start. There are cases of people sitting down and writing a good book just like that, but there are also cases of people being born with two heads. It's overwhelmingly likely that you would need to write, solicit feedback, write more, write again, and keep doing it until you got good enough for people to demand your work rather than suffer it. Short stories are a great medium to improve your writing in as they don't require months or years of effort on the writer's part and only require minutes rather than days from the reader. I found it important to see the impact of my efforts so I knew what worked. So I joined online critique groups and shared short stories. I did that for for a little bit, wrote a bad book, did it for several more years, wrote an OK book, did it for a couple more, and wrote Prince of Thorns. And before writing my first short story I had been writing in various other forms for decades.
Essentially it's a labour of love. If you don't love writing - just for its own end - then you're probably better off doing something you do love.