A pool is worth whatever a buyer is willing to pay for it. A pool is of no value to a buyer who does not want a pool but can be very valuable to a buyer looking for strictly pool homes. To assign a dollar amount to a pool would also depend upon the design, age, functionality, location, etc. Yes, a spa can be a great additional to the pool and add more value to the property.
It can cost upwards of $100,000 to $150,000 to install a pool and spa depending on the design, etc. It is unlikely that you will get a dollar for dollar return on the cost of your pool. A buyer looking for a pool home in your neighborhood will be willing to pay an additional $25,000 to $40,000 (maybe) to alleviate the headache of having to install one themselves. But there are so many factors that come into play I cannot give you a definitive answer and please don't bank on my "blind" estimate quoted above either. An experienced Realtor in your area will be able to assess the value more accurately for you. Good luck !