delta neutral option cost

If you sell a call option, in the language of vol trading you are short gamma and vega, and long theta.

So yes, as the underlying wiggles around, if you keep delta-hedged by trading it you will lose some money. This can alternatively be seen as gamma losses (coming from the second-order move in the underlying, which you have not hedged) or as coming from the hedging strategy.

On the other hand, you are compensated for this by the time decay of the option price, ie. its theta.

By the end of the option lifetime, if realized vol over the period (which is effectively what you pay out to hedge) was higher than the implied vol price you received for the option (which was the premium you received), you probably* lost money, and if it was lower then you made money.

*this isn't exactly true due to vol itself being stochastic... here is a great reference on the topic Wilmott, Ahmad: Which Free Lunch...

EDIT

Responding to comment: The first section of the wikipedia page does a good job answering this... if we re-express BS equation as begin{align} {frac {partial V} {partial t}} + {frac 1 2} sigma^2 S^2 {frac {partial^2 V} {partial S^2}} = rV - rS {frac {partial V} {partial S}} end{align} then you see that the term on the right (risk-free growth coming from a portfolio of the option and the short delta hedge) exactly cancels the term on the left which is the gamma plus the theta.

So we can hedge away delta, but we are still exposed to these two.