AJD- If you are a licensed grower or have a proper farm business, I don't see why a copacker wouldn't let you bring in your own produce. If you were just a home grower, with no licensing of any kind, yea, I can see why they wouldn't be thrilled about letting you bring in your own produce. Sea Fire (formerly Davez Foodz) goes to eastern Washington in the fall, they help pick something like 50 cases of a couple different chiles, take them to the co-packer for processing in their sauce.
What they charge?
I got a quote from one copacker who wanted to charge $600 to review my already-approved process authority letter and recipe/process.

WTF? It's BBQ sauce, throw the stuff int he pot, cook it up, bottle it! HELLO?!?!? it's not rocket science~~~
Anyway- a lot of copackers don't have time to babysit new clients. Your best shot would be to get your recipe dialed in, see what the ingredients are in the final version, then look at what type of processing would be appropriate. Does it need canning or is hot packing OK?
Some copakcers will walk you through everythign from start to finish including recipe development, but it'll cost ya!!! Their time ain't cheap. The more you can do ahead including getting the recipe finalized and approved by a process authority, the better it will be to talk to copackers.
That again will have a lot to do with how much tehy will charge for a run. Are you hand pealing 20 pounds of carrots, 15 pounds of onions, roasting and pealing 8 pounds of beets, roasting/blackening 40 pounds of poblanos and then peeling.....
Or do you toss it all through a grinder, splash some vinegar on it and cook it up?
You see where this is going....
I'd say, get together with your friend, spend a year making test batches, then make another, and another, and another,and another.....By the end of this coming winter, you will know what produce you need to grow and you'll be ready to take the next step.
Good Luck and HAVE FUN!