There are many aspects to this. Normally, packaging facilities charge you per shift, per line, that is, 8 hours or so. They put stuff in a jar, put a label on it, and pack it into boxes. Different facilities have different kinds of equipment and different line speeds and of course different capacities. They also have different kinds of standards. For example, some lines may have metal detection devices, or some lines may only be able to handle what they call drop weights at 0.5oz increments. All these things vary, and there are tons of places out there. You just have to find one which fits your needs. If you're canning, you have to be USDA certified. Period, or have someone who is certified to go over the specs. This is about cook time, water activity, acid, etc. Food Science 101 covers some, but not all of it.
Second, the ingredient deck. You will have to find someone to go over the ingredient deck. There are laws about what's legal and what's not, and what you must declare on the label.
Next, you need a tracking procedure in case there's a recall.
Next, non-disclosure agreement. Find a good attorney.
Next, your specs, the contract, and how you will enforce them. What's your responsibility, what's theirs. Never leave this to chance or let the quality of your product hinge upon a simple handshake. Storage issues, for example, of raw product. Can they store your volatile product, (normally oil) under a nitrogen blanket or will it sit out in a storage tank under the sun for two days?
Lots of things to think about.