If you’re really hard up for caffeine, purchase pure caffeine powder. $20 is enough to satiate an average person for a year, or to overdose daily for a month.
Looking at the math it honesty looks like cheap black tea and cheap caffeine powder are about the same cost. Can get 400 tea packets for 15ish bucks, or a year supply for 15$.
Please for the love of god be careful if you do this, buy some quality scales that can measure mg to a fair degree of accuracy or stick to quantified amounts you can purchase.
A friend of mine used pure caffeine powder in his protein supplements and ended up overdosing one night, it's a horrendous way to die.