There’s two possible solutions to this:
1. All posts have meta
The best solution I have found here is to give the rest of the posts/products an item price of 0. You can do this manually, or loop through all the posts and if the price is empty then update it.
To make this manageable in the future you can hook into save_post and give them a value when they are first added (only if it’s blank).
2. Multiple Queries
You could run the first query as you’re doing and store the IDs of the posts returned. You could then run another query for all posts and orderby date, excluding the IDs return from the first query.
You can then print out the two results separately order and you’ll get the desired results.