Estimation of Plan quality

Dear all

Can someone tell me how to estimate the quality of my plan. For example if
the ideal query plan generated by Dynamic Programming has a plan quality of
1.0, and suppose I generate my query plan with some greedy method or
something, it will be worse (not as optimal as) than ideal plan by, say,
1.5 or 2.0 times , how can I know this factor?

Should it be estimated from the execution time?

In the plan generated by PostgreSQL when I give explain query, I see Plan
cost at the top of the plan with some exponential value. What does it
indicate? Is it the estimated execution time? or Is it the search space that
has been considered during plan execution?

Thanks
Vamsi