Like Gantz for example has characters with realistic faces instead of big eyes and hair colors but to use to this can be expensive so why is it cheaper to use anime and manga faces that lack realistic features like characters with no visible lips?
Like ton.yeung said, more detail takes more time and thus costs more money. In theory you could make an anime with every single frame having the same level of detail as a painting by Leonardo da Vinci, but it would be prohibitively expensive; it took Leonardo years to finish a single painting, and you would need a whole team of Leonardos churning out thousands of them in a limited timeframe.
That said, I doubt that it was significantly more expensive to animate Gantz than, say, K-On. The question seems to presuppose that cost is the only thing keeping anime from having more realistic facial art, but I believe it's more about artistry and style. The experience that Gantz was trying to create was drastically different from the experience that K-On was trying to create.
The K-On characters are lacking some facial detail compared to the Gantz characters (in line with well-known rules of cuteness that, yes, seriously, have been studied and derived by anthropologists). Gantz uses smaller eyes, chunkier bodies, and a more subdued color palette; this makes it seem more realistic. However, we can see that both of them are lacking a lot of detail compared with, say, the work of American comics artist Alex Ross.
(I should note that Ross is primarily a cover artist, because of the time it takes him to produce works of such high detail. To create an animated feature at Ross's level of detail would be untenable.)
Also, shows like K-On and Clannad often have very detailed clothes, backgrounds, and other objects. Look at the instruments in K-On.
Look at the detail in the background of this random screenshot from Clannad.
So I don't think the difference in style is primarily about cost. It's about artistry. K-On was well enough funded to make its art look like Gantz had the creators wanted to; but that art style didn't fit with the goals and aesthetic sense of the series. It's the same reason Bugs Bunny looks like this:
and not like this:
While the typical anime art style may have originally been created as cost-saving measure, that seems to be no longer the case. It is recognized as a unique style on its own and appreciated by many people on that basis, the same as the highly unnatural Cubism and Mannerism (which produced what seems to be the first known forerunner of the Shaft head tilt).