Packaged blood won't work here
Tainted Sorcerer says:
She has two choices for a source of blood: herself or someone else.
If she uses her own blood, the minor knife cut to draw the requisite blood is a free action that becomes a normal part of casting the spell. (...)
If the tainted sorcerer draws someone else’s blood to use as a material component, the rules given above generally apply.
The blood must be obtained freshly by cutting a creature as part of casting the spell. So packaged blood as a good that the feat applies to won't work.
You however could still power your spells by blood you bought, if you lugged around a proverbial sacrificial lamb.
Blood prices would depend on the kind of blood
Even when blood is not listed in the equipment list, you can buy blood by buying livestock (listed on the trade good table, or on the table for mounts). This also has the advantage that if you bring the creature along, you can use it as a source of blood for your tainted magic. The price would depend on the animal.
For example, a goat costs 1 gp, and has about 3.15 liters of blood, about 0.8 gallons so goat blood would cost you roughly 1 gp 2 sp per gallon. As the cost is measured in hp, not gallons, a goat has on average 5 hp, so a cost of 2 sp/hp.
A pig costs 3 gp, and has about 7.15 liters of blood, costing you about 1 gp 5 sp per gallon, with an average of 6 hp, of 5 sp/hp.
Human or elf blood may not be available to purchase this way, or at least would be evil or illegal in campaigns that encompass slave trade or human trafficking. Thanks to @From, I can report that "Slaves are costly (a young human field slave sells for about 50 gp in the markets at Eltabbar), so few commoners have the means to own them." - Unapproachable East, p. 159. A human would have up to 1.5 gallons of blood, for more than 33 gp per gallon, and a first level commoner has a d4 hit points, so on average 20 gp/hp.