In the Seachem lineup, they sell Flourish Nitrogen, Flourish phosphorus and Flourish Potassium. 3 separate products.
That said, compared to mixing your own from dry ingredients they are substantially more expensive.
Seachem nitrogen is made from Potassium Nitrate (KNO3) and urea. It is 1.5% nitrogen and 2% potash (K2O). 2L of this mix costs $32 over on Amazon, and provides 30 grams of total nitrogen (1.5% of 2kg). About $1.06 a gram of nitrogen, assuming you get free shipping.
A pound of KNO3 costs $3 (+$6 shipping if ordered alone) over in the for sale area here and is 13% nitrogen. 453 grams * .13 = 58.89 grams of nitrogen. 15 cents a gram of nitrogen, even including shipping. 5 cents a gram if you don't consider shipping costs.
Do you see why pre-mixed macronutrients aren't very popular? Packaging and shipping water around gets expensive.