Table order at FROM clause affects performance?

I have a strange performance issue, i think that is not possible:

Given this statement:

SELECT *several_fields* FROM A, B, C WHERE *conditions*
A, B are tables with several LEFT JOINS but they act as one subquery.

If I execute the select above:

SELECT *several_fields* FROM A, B, C WHERE *conditions*
*Time: 30 secs*
*Cost: 1M*

If I execute the same select (same parameters) but swapping A and B in the
from clause:

SELECT *several_fields* FROM B, A, C WHERE *conditions*
*Time: 19ms*
*Cost: 10k*

The plan changes dramatically: I can't see why the order of FROM clause
impacts directly on the query cost and plan. If this is possible, where i
can read about it? I need to know how the order of FROM clause modifies the
query plan.

Thanks in advance. This is my first post.

Eduard Català