Force PostgreSQL to query a custom index structure

Hi *,
I have a question according to a customly built index. I want to force the query planner to search in my index structure, whenever I have a query that uses my specified operator. Until now, I always have to insert a quite amount of tuples (~ 20000-30000) for being able to get an index scan instead of a sequential scan.
What I tried here is to set in my costestimate function:
*indexStartupCost = 1;
*indexTotalCost = 1;
*indexSelectivity = 1;
*indexCorrelation = 1;
However, this does not work, apparently. I still have to put a quite amount of tuples to the table and build my index on top of this. My costestimate function is called properly and returns these values. What do I have to adjust in order to force the query planner to use my index structure (which would be much easier for me to test) and not having to insert a real big amount of data items?

Best regards
Carsten Kropf