whether it is cheaper or not depends on several of factors. For every 10 ppm FC added by trichlor, 6 ppm CYA is also added. You will also lower he pH and TA somewhat so might need baking soda and/o borax. If you must add CYA it will have no noticeable impact on your current water balance. IF you already have the trichlor on hand go ahead and use it but if you need to buy it using CYA is cheaper in many cases.