If I had a 15 ct Alex with a good red to green color change, I would not try to establish a valuation myself. Instead I would send it to one of the premier gem labs for identification and then valuation. Unless you dug it yourself, there is certainly the possibility that it is synthetic. Even if you are sure it is natural, a buyer (or insurer) would want proof that it is a natural stone in the form of a reliable ID report. Beyond authenticity, valuing a very rare stone is tricky and you need an appraiser who knows the current market (which might consist of very few people) and recent sales. Working from a price guide you run the risk of significantly overvaluing or undervaluing the stone.
The price will also depend on what market you want to sell it in. If it is an auction sale, that is one thing. You find an appropriate auction venue and roll the dice. If you would sell it to a jeweler that will be closer to a wholesale price and there are only a relatively few who would want such a stone. If you are looking for a private retail sale, you may need someone to sell it for you, need to wait a signficant amount of time for the right buyer to appear, or need help from someone who knows high end collectors who might have an interest in such. An expert who calls a collector and vouches for a stone could be the magic ingredient that seals the sale. You might even want to consider partnering with a famous jewelry designer who would design a piece to showcase the stone to increase its sale price. This might involve hand wrought platinum and a number of really nice diamond melee to surround this prize.
On the other hand, if the color of the stone has significant brown tones that detract from its appearance or significant inclusions that reduce its brilliance, it could be worth a lot less than you think.
Short answer is, don't try to value any stone that is rare or expensive unless you are an expert. Spend a few percent of the expected value to get an accurate estimate of what it's worth. And consider getting help to sell the stone from an expert, whose commission might be a worthwhile expense that increases your profit in the end.