Doesn't matter if it was cheaper, the taxes already existed. The real problem was that Tea Act granted a monopoly to the British East India Tea Company by undercutting any competition.
It was a government sanctioned monopoly to bail out the Tea Company.
As in, you have to pay the taxes and you have to take these shipments from us.
It doesn't MATTER if it was cheaper than other goods, it was the fact that they had to buy and pay taxes on THOSE goods whether they liked it or not. When they tried to send shipments back, they weren't allowed. That's what lead to the Boston Tea Party.
Yes the tax was an issue, but it was the tax combined with the fact that they were destroying any chance of competition with their Monopoly, by selling tea at undercut prices and not allowing the shipments to be turned away.
The tea being cheaper was actually part of the problem.