I had a really good look at Whoa Nelly (inside) and as much as I really like the game, I would be surprised if it cost more than £1K to make, so with modern assembly techniques, and reduced cost serial wiring that everyone is going with, why would an old single level Bally/Williams not be doable for the same?
Look at the (ex vat) price that Heighway games are selling their Full Throttle at, compared to a Stern pro, bearing in mind that Full Throttle looks significantly more advanced compared to your current Stern Pro cost saving games. Assuming Andy's business plan has enough profit to earn a living and keep his factory going, it shows that even advanced games can be made for a reasonable price.