Hi James,
Best of luck when getting quotes for this. It's really going to be across the board in terms of pricing based on your particulars.
When you're at the point of searching for an integration solution with eBay, feel free to let your developer or web development agency know about our Shopify connector which integrates your products with your eBay and Amazon accounts: https://www.nchannel.com/connectors/shopify/
Good luck.
Mollie
nChannel
