costing for arbitrary path

I wish to estimate the total cost for a given path of a query. This
path is a tree consists of nestloop, hashjoin, mergejoin as internal
nodes with seqscan or idxscan at the leaves.

My approach is to cost the path bottom-up, and invoke the
"cost_nestloop", "cost_merge_join" etc.

The problem is that each join method has some value that seems to
change value each time I cost the SAME path. For example, the
hash-join has "thisbucketsize" varying from time to time. I noticed
this is because the caching the bucket size estimate inside the
RestrictInfo structure. Is there a easy way to cost a give path?