This thread is a companion to the Cost Comparison of Chlorine Sources thread here. This thread compares the costs and pros/cons of different methods of adding Cyanuric Acid (CYA) to the pool.
Cyanuric Acid is sold at Leslie's online here for $36 for 8 pounds or $4.50 per pound [EDIT] see posts below for prices as low as $3.10 per pound [END-EDIT]. It is nearly 100% pure, but dissolves very slowly and is somewhat acidic. If put into a sock or panty hose and hung over a return flow or put into a skimmer sock in the skimmer (with the pump running 24/7) it can dissolve in a day or two instead of a week [EDIT] see posts below as it can dissolve faster than that if physically squeezed and helped along [END-EDIT]. One pound of CYA increases the CYA by 12.0 ppm in 10,000 gallons.
Natural Chemistry's Instant Pool Water Conditioner shown here has one gallon add 40 ppm CYA to 10,000 gallons. It is sold at InTheSwim online here for $25 per gallon. It readily disperses into water and is relatively pH neutral -- slightly alkaline.
From the chlorine cost comparison thread, Trichlor is $2.20 per pound while Dichlor is $2.60 per pound. One pound of Trichlor increases the FC by 11.0 ppm and the CYA by 6.7 ppm in 10,000 gallons. One pound of Dichlor (dihydrate) increases the FC by 6.6 ppm and the CYA by 6.0 ppm in 10,000 gallons.
So the cost per 1 ppm CYA in 10,000 gallons is as follows for each source:
Cyanuric Acid ........................... $4.50 / 12.0 = $0.375 [EDIT] $3.10 / 12.0 = $0.258 see posts below [END-EDIT]
Instant Pool Water Conditioner .... $25 / 40.0 = $0.625
Trichlor ...................................... $2.20 / 6.7 = $0.328
Dichlor ....................................... $2.60 / 6.0 = $0.433
I need to add the cost of Borax to compensate for the acidity of Cyanuric Acid, Trichlor and Dichlor (I'll edit this post later), but essentially you can see that Trichlor is least expensive and you get addition of chlorine as well, but it is slow to dissolve. Dichlor is fast dissolving and you also get chlorine so is probably the most economical way to quickly add CYA.
For the 1 ppm CYA rise in 10,000 gallons, Cyanuric Acid needs 1.7 ounces weight of Borax to maintain pH. Trichlor, after chlorine gets consumed, needs 8.0 ounces weight of Borax to maintain pH. Dichlor, after chlorine gets consumed, needs 3.8 ounces weight of Borax to maintain pH. I found 20 Mule Team Borax for $4 per box (76 ounces) on Amazon. So one needs to add $0.09 to the Cyanuric Acid price, $0.42 to the Trichlor price, and $0.20 to the Dichlor price for a more accurate comparison.
If I use Arm & Hammer Super Washing Soda instead of Borax, then Amazon sells a 55 ounce box for $2.70 (shipping is high, but some grocery stores had it for this price as well). In this case pure Cyanuric Acid needs 0.86 ounces weight of Washing Soda, Trichlor needs 4.0 ounces, Dichlor needs 1.9 ounces. So one needs to add $0.04 to the Cyanuric Acid price, $0.20 to the Trichlor price, and $0.09 to the Dichlor price.
Another alternative for raising pH and similar to Borax in not raising the TA as much is Lye (Caustic Soda) which can be obtained for around $3 per pound. For the amounts as indicated above, pure Cyanuric Acid needs 0.35 ounces weight of Lye, Trichlor needs 1.6 ounces while Dichlor needs 0.77 ounces, so this adds $0.07 to the Cyanuric Acid price, $0.30 to the Trichlor price and $0.14 to the Dichlor price. So this is less expensive than Borax, but more expensive than Super Washing Soda.
To be fair, we should subtract the price of the least expensive chlorine from the Trichlor and Dichlor since you do end up adding both chlorine and CYA with these sources. From the chlorine cost comparison page, the cheapest source of chlorine was Trichlor (not counting the Borax required to maintain pH) so essentially using Trichlor pucks/tabs to add CYA it costs incrementally $0.42 which is still higher than pure CYA. Dichlor incrementally costs $0.433 - $0.328 + $0.20 = $0.31 (or $0.20 if using Washing Soda for pH adjustment) so not that much different than pure CYA (depending on where you get it).
So considering everything, both pure CYA and Dichlor are reasonable alternatives, with the latter being reasonable when you aren't in as much of a rush and can increase the CYA as part of regular chlorine addition (or for shocking, if that was to be done anyway). Of course, going away on a trip and using Trichlor pucks in a feeder also work (more expensively) if one is not in a rush to increase CYA quickly. Sounds like a personal choice to me.