does it logical to invest on expensive transport?

it's all relative to how much money you have or your intensity of interest

Logical? Maybe, or maybe not, and I agree with Larryken. It depends upon the level of your system. IME, a transport is even more important than a DAC, and a computer is a transport FWIW. At 15k you are at the end of the line where computers meet high end transports in performance. I'd opt for the computer, because it will be much(5-10x) cheaper. I wouldn't be surprised if the right transport could best the best computer set up though.

It might not be long before computers catch up though...

I definitly desagree with 4est. numbers are numbers, it is the conversion of those numbers that matters. Only since the improvement of conversion(dac) has the digital domain began to show it's potential. What is possable. Besides, if you download there is no need for a transport. I have down loaded several 96/24 recordings. They sound great. Are they better then every cd I have ripped, NO! But most. If you have a lot of sacd's I understand your intrest in a player. But if you have redbook, move on. You will not beat it. And, in the end, you will be much happier with your system. MY OPINION!

Yes, absolutely a really good trasnport will improve performance. Absolutely no question about it. Anybody who says that it won't hasn't ever heard a good transport vs. a average one.

Are hi-res digital files better than the CD? No, at best, they are the same as the CD from which they were/are copied.

Can a server based source compare to direct from CD? Yes, with a good DAC.

Should you buy a really good transport? Depends on what you see for your future. Server based systems are very convenient and done right with a very good DAC can be truly great.

Teajay makes a good point, if you have a large collection of CDs that you prize and have no plans to download, a high quality transport is mandatory for optimal redbook performance. It`s amazing just how good redbook can sound in a well thought out system, old fashioned? but still excellent sound without question.