My $0.02, for what its worth.
From the insurance company's standpoint, they need to know what it will cost to replace the cue, just like theft of a car. If you have a receipt from a dealer and show that as proof of the value of the cue to the company, I would assume you would get that value minus any deductibles applicable. I don't think there would be any depreciation.
As is the case in many instances, your question would be the same if you had traded to get the cue. How would you be able to prove to the insurance company that your cue was worth what you claim it to be. The easiest way would be to go back to the manufacturer or establishment where you bought the cue and get them to put in writing what the cue would cost to replace. The claims adjuster may also surf the net to find out what they could buy the cue for, and give you that value. I would think that they may even be inclined to look on the "sold items" on ebay for your particular brand of cue, if you are claiming any substantial amount that they may have reason to doubt. The average Joe on the street probably wouldn't believe you when you tell them that you have a cue costing anything over $1000. Thats why you would want to inform your agent, in writing, before any of this occurs. He would be hard pressed to deny you that value if cues are covered under your policy.
Also, you can find the rough retail value of any cue by surfing yourself on a number of websites. Some of my favorites include:
www.cueaddicts.com
www.classiccues.com
www.cueauctions.com
www.waltoncues.com
www.iconcues.com
www.cornerstonecustomcues.com
www.billiardcue.com
www.onthehill.net
www.billiardwarehouse.com
www.hawleys.com
For those of you that I missed, my apologies in advance.