Estimated Cost to Restore my Dad's Deville

Based on your pics you have a nice De Ville, most probably dating to the mid 1960s.

I think the dial is original, Omega had many variations of the Seamaster/De Ville around the time of transition to plain De Ville.
The movement is a caliber 711, the thinnest automatic caliber ever produced by Omega.

First thing is to remove the flexi bracelet. The spring loaded end links can mark the inside of the lugs, not so much with stainless steel but badly with gold.

Polishing the case tends to soften or even remove the sharp facets and edges, especially on the lugs. It can also remove the gold plating and even gold cap if done by a ham fisted "restorer". It should just need a cleaning with mild water/detergent solution and a light polish with a jewellery cloth.

A good independent watchmaker in the SF area should be able to bring this watch back to life, from what I've seen of US prices, maybe $300 or so? Just don't send it to Omega, it would cost more than the watch is worth.

Maybe some US peeps can give you some pointers on a good watchmaker.

With a service, a clean and a nice tan leather strap your Dad will have a very nice watch.

To restore this properly it could be quite expensive. If you look at the photo of the case back, there is evidence that that the rotor is rubbing on the inside of the case back, so the rotor pinion is worn. If this were a Cal. 55X then the pinion in the rotor could be replaced for a small amount, but the 711 does not have this part available. If this is still in a worn state, a new rotor alone would be $300. Usually if the pinion in the rotor is worn, other parts will be as well from someone wearing it without regular servicing.

No one is going to be able to give you an accurate price on the forum - best you can get is a ballpark that might be a home run, or a foul ball...

You need to take this is a watchmaker well versed in vintage Omegas, with access to parts, who can give you a price.

Cheers, Al