It really depends on the pool and how involved you are going to be. If you are going to check the water and have a vacuum do regular sweeps you can expect to pay less. Think of it like a car - keeping it up and making sure you do preventative maintenance makes all the difference.
I would say in general, expect to add $100-$300 to your monthly bills (water, energy costs, pool chemicals and/or someone to come and clean). This is a general, monthly average with the understanding that some months you will be using the pool more than others.
Something to know about the buying process - Once you have made an offer and the seller has accepted your offer, you have a disclosure period. This period is when you get information about the property from the seller (among other things). Aside from the standard questions asked of the seller during the sales process on standard forms called disclosures, you can ask them about other things that are important to you.
You can ask how much they pay for utilities on a monthly basis and what they do to take care of the pool. They likely won't know how much of their energy bill goes towards the pool, but finding out how often they maintain the pool and do they heat it or not, will be able to help you adjust your estimate of how much it will cost you on a monthly or annual basis. A good Realtor will formally write up the request for information along with a short explanation of why this information will be helpful and submit it to the seller on your behalf.
If you have not yet found yourself a Realtor you have a good relationship with and trust to educate you about this process, I'd be delighted to sit down with you and let you know how I work and how I may be of service should we be a good fit.
Hope that helps,
Gloria Gonzalez, Realtor