Ignoring the difficulty set by the pool Scenario:


The pool sends a mining.set_difficulty message with difficulty = 8.
The miner ignores the message and submits shares that qualify at
difficulty = 1.
Most shares will be rejected by the pool with error = high-hash.


Question: what is the cost of submitting shares that get rejected? Are the accepted shares as valuable with an acceptance rate of 5% as if they were submitted with an acceptance rate of 100%?
The pool will typically ignore invalid shares. They should have no effect on shares that are actually accepted.

Some pools might consider excessive submission of invalid shares abusive, especially if done at high rates. The point of an increased difficulty is precisely to avoid this. So you might find you get suspended or disconnected. But most pools would probably just tolerate it unless it actually caused them an operational issue.