SET EXPLAIN - why are INSERT costs high???

We are stress testing our application and have been pouring over the SET EXPLAIN OUTPUT to optimize things. Everything looks good but the INSERTs are still reporting high costs. My theory is that it is handling the INSERT like a SELECT without a WHERE clause. Our hardware vendor has said that we need to reduce these costs before they will tune our database. I don't think it is possible. I don't think the high costs represent a failure to optimize. I am very sure about this. Does anybody know the scoop. If so, is there any reference in the Informix documentation that I can go to. It seems like indexes should have some impact on the efficiency of doing inserts. Does SET EXPLAIN help cost this at all????? thanks, dave k