No, it costs double to move through a threatened square
This was clarified on the 3.5 FAQ, which the community has mixed opinions about (as seen on this other question):
For example, imagine Ember begins her turn 10 feet from an orc and wants to use Tumble to move through its space and end up 15 feet away on the opposite side.
Her first square of movement wouldn’t provoke attacks of opportunity (since she’s not leaving a threatened square), so she doesn’t use Tumble or move at half speed.
Her second and third squares of movement take her through the orc’s space, so these effectively cost twice normal (that is, they each cost 1 extra square of movement), and this movement requires a DC 25 Tumble check. (If you’re using this system, it’s much easier to think of tumbling as costing extra squares of movement rather than actually changing your speed.) Ember has now paid a total of 5 squares of movement (out of the 8 allotted to her for her speed of 40 feet).
Assuming that check succeeds, Ember now moves from a threatened square (adjacent to the orc), which requires a DC 15 Tumble check and costs twice normal (or 1 extra square of movement). She’s now paid a total of 7 squares of movement.
Finally, Ember moves one last square, ending up 15 feet from the orc on the opposite side from where she began. She’s not leaving a threatened square, so she doesn’t use Tumble or pay any extra movement. Ember has paid for 8 squares (40 feet) of movement, and has covered a linear distance of 5 squares (25 feet).