Some factories will try to take advantage so if possible get someone in your area from a fashion council or business start-up centre etc. to help from the beginning. They may also have a list of factories and know which one is suitable.
You may need to choose several factories to produce one collection. One will have machinery specializing in stretch knits for example so you will take your jersey items to them. Another may do coats, or shirts, or jeanswear or specialize in difficult fabrics like silk. Not every factory can do everything, and some may be very basic e.g. lockstitch, overlocker and buttonholing machines so they won't be able to cope with anything too complicated (even though they may still want to take on the business!)
It's also wise to choose several factories as this gives you a better chance of having your collection finished on time. Relying on one manufacturer is dangerous.
Your patterns need to be cut from card. All the usual markings need to be on them, and hiplines etc. should be notched, bottoms of darts & pocket positions drilled hung then punched & hung on a hook with any special instructions or working drawing attached. A sample garment can also be included. Try to make things as foolproof as possible for them as mistakes mean more time and are costly! They like things to run smoothly around the factory and patterns that need a bit of tweaking (the way we might do if making something at home, easing in a collar that's a little too big, for instance) won't do in a factory environment.
For a factory, it all comes down to time. If you want a hem that's overlocked and stitched up this will obviously be cheaper than finished with bias-binding then blind hemmed...overlocked seams as opposed to French seams, 5 pockets instead of 2. It's ultimately up to you. You are the customer, and if you are intending on having it manufactured locally, many factories will be very helpful....it's not a great time for them at the moment and they are learning to appreciate business!
Being able to cut your own patterns and make your own samples will mean you know what you're talking about when telling the manufacturers what you want, decreasing your chances of being taken advantage of
Sorry if this is all a jumble. I will try to reply later with some more stuff!