It is. But it's still not as "expensive" as pregnancy for a woman. I use scare quotes because I don't just mean monetary costs, but all kinds of costs.
You're still missing the benefit side of the cost/value equation. It might not seem like this to you but having a child is a major value to many people.
A woman has multiple options to prevent unwanted children (including two after the fact options). She therefore has full control of the child/no-child outcome based upon cost/value for her personal situation. A man has two options to prevent childbirth: a permanent vasectomy or using a condom (these fail and they fail too often in real world settings). A man has limited (or in the case of a condom mishap potentially no) control and potentially if the woman decides it should be so no value whatsoever.
I completely understand the upside. I am, however, assuming a situation where both parties do not want to have a child, right then. The original scenario was, implicitly, casual sex. (Original scenario: "her: do you have protection? him: um, sure. I'm taking that, uh, that pill thing.") We can be more charitable and assume that scenario could play out a few months into dating. In such situations, not wanting a pregnancy is generally taken as a given. The entire discussion - birth control - even assumes that pregnancy, at the moment, is not wanted. So I'm confused why you're bring up the value of having children (which I do not deny) in a discussion about the incentives for men and women in avoiding unwanted pregnancy.
Perhaps your confusion is that I stated "There is no incentive for man to impregnate a woman", and you interpreted that as a universal statement. It was not. It was a direct response to the question for a man's motives in the given scenario, which assumes unwanted pregnancy.
The point I'm trying to make it that the cost is not the only part of the equation and that we must also account for reward and control.
Cost/reward
When the female birth control pill became available the cost/value was weighted very heavily against the woman and female control was much more essential. However today the cost/value is more evenly weighted. Back then a man could deny everything and move on leaving the woman with the baby and a ruined life. Now the man can't deny everything and as a minimum will be committed to child maintenance but he may still be denied access to the child. So the situation now is: woman high cost coupled with high reward; man moderate cost and potential reward controlled by the woman.
Control
Let's assume that both parties decide for themselves based on their own cost/reward situation. Let's assume that they don't always inform the other of the their decision honestly. Neither is perfect judge of character.
Current situation
Scenario 1: Man wants child. Woman does not. Woman controls outcome. No child.
Scenario 2: Man doesn't want child. Woman does. Woman controls outcome. Child.
Scenario 3: Both want child. Child.
Scenario 4: Neither want child. No child.
Situation with male birth control
Scenario 1: Man wants child. Woman does not. Woman controls outcome. No child.
Scenario 2: Man doesn't want child. Woman does. Man controls outcome. No child.
Scenario 3: Both want child. Child.
Scenario 4: Neither want child. No child.
P.S.
Personally I think that the pill was one of the greatest drivers of women starting to have rights on a par with men (I'm not saying that 2015s western world is perfect gender equality wise but it is a lot better than the 1950s western world was).
P.P.S.
I just think that men should if possible have the option to control their reproductive function and that it does not matter whether woman trusts the man -- the man's control does not affect the woman's control (except in the scenario that the woman is a deceptive actor - NB that the inverse situation where the man is a deceptive actor is already under the woman's control).