Just went through this last week...Our local Home Depot only has the "Green" version, which is around 20%. Our local Lowes has the Green (20%) and the regular (31%), the latter was about $8/gallon. Menards has both as well, and their 31% was $6/gallon, however they were sold out (and why I ended up at Lowes).
In the Southwest markets Lowe's and Home Depot only carry 14.5%. When they switched from 29% to 14.5% they never lowered the price. They continued to sell the half concentration at the same price as before. I get my 29% or 31% at Leslie's.
In Florida I get it at Home Depot a little over $11 for 2 gallons. I'm using less these days after implementing some recommendations from others here such as letting my TA drop to 50 and holding the pH around 7.7. I use about 12 oz twice per week now.