We have an inground 30,000 gallon salt water play pool and a hot tub combined. (The hot tub did not originally come with the pool we added that after 1 year with the pool). If you buy a salt water pool you still add shock and other chemicals to the pool. Honestly I spend right at $80 a month in the swimming months plus extras for the Polaris ( the robot cleaning the bottom of the pool 8 hours a day). He is in the water all the time- his tail needs to be replaced about every 4 months -$10 a piece and the bag gets nasty/ripped and I buy one of those about once or twice a year -$35. He really needs to be serviced about once every year or two (maintenance is the key to anything that works over 8 hours a day)- That cost around $150. If he breaks and it's beyond repair they range for a good one $800 to $1500.
My pool guy has told me it cost around $100 a month just for the pool pump to run. I figure we range $100 to $150 a month because we have a water fall that requires an extra pump and we are outside every weekend by the pool. Since we added the hot tub we have propane to heat it up- we use propane for the hot tub, stove top and fireplace. We spend about $600 to $800 on propane a year. I don't feel that is to much as it really helps us utilize the pool a little longer as it's fun to jump in the cold water and then warm up in the hot tub or vice versa. =) And when it snowed 14 inches...the hot tub was AWESOME!!
I don't pay for a pool guy- the ones i have checked on will not vacuum the pool or backwash the pool. (On salt water pools you have to clean another internal filter that uses Dio Tenacious Earth which is the backwashing of the filter) So basically I found that they were charging $75+ a month just to empty strainers and the Polaris and sweep some of the dirt to the center filters. I enjoy being outside and doing those little things as it only takes a few minutes and we have trees so I would have to do it everyday anyway if I was going to swim so it seemed like a waste of money to me.
Oh and don't forget your water bill..it's hot in Texas and you will have to add water. In the summer our water bills range from $100-300. Keep in mind that is water the yard as well but you can figure at least $100 to $200 for 2 or 3 months out of the year.
So to answer your question I would guess the pool cost me an additional $3000 a year. Granted it's not all at once but realize the pool will bring your family and friends together. We spend every minute that we can in the back yard. We have a A/C company so our summer months are crazy busy and it's nice to escape to our own backyard oasis and have daily mini vacations and still be at home!
My kids are 5 and 7 and we have been in this house for 3 summers now. My 5 year old started out as a fish and my 7 year old was scared to death of water when we first moved in. This summer he started swimming with out floaties and under water. I think buying a house with a pool was the best investment I have ever made. I LOVE IT!!