Good afternoon!

I have installed and I'm using latest Neon User LTS Edition on my amd athlon 64x2 5200+ with 2 gb or ram

I'm impressed by how the whole system is quite a lightweight one, while Amarok takes 400+ mb of ram (and almost costantly 10-15 % of cpu usage) as soon as I open it, no matter which phonon backend I use (vlc and gstreamer gave me the same result) or how many plugins, script and various functions I disable.

I had the same results on other (live) distros, which means I didn't even let it scan a collection, like kubuntu 16 and mint 18 , all 64 bit. same results on different systems too

To me it really lloks like a bug but I just wanted to know if it's a normal behaviour

Thank you anyway

Are you talking about global RAM or are you talking about the residual memory? Remember, memory allocation in Linux is dynamic.
I have Amarok running under Kubuntu 16.10 right now and it uses 156 MB residual RAM, running since 2 days. That is certainly not too much for a GUI application, every browser uses at least as much as that.

I'm talking about the amount of memory on ksysguard: for amarok it's about 430 for the "memory" column and about 110 for the "sharted" column (and again, 10% of costant cpu usage, even if it's not playing anything). What do you mean with residual memory?
What suprised me is that I've never seen such a behaviour in any other similar software (rhythmnbox and tomahawk are both around 100-150 mb of ram usage and no costant cpu usage), not even on firefox which, as a browser, gets 230mb of ram memory before opening a webpage, and still no constant ram usage.
What surprised me even more is to see such a behaviour in such an overall, quite lightweight desktop environment. I mean: 430 mb of ram usage is MORE than the WHOLE kde system without any other software!

I doubt you really can compare applications, as neither Rythmbox nor Tomahawk have the same structure as Amarok. The residual memory I was talking about is actually the shared memory in ksysguard, and that is the only one that matters. So one should always only consider the shared memory, as the global memory use is dynamic and very much depends on what else the system uses.
As for the discrepancy in memory with the desktop you run, this is easily explained by the fact that your Amarok version is quite old and still uses Qt4, so it is not optimized for a Qt5 based desktop.

The odd thing is that I installed latest opensuse to check and it didn't have the same problem as the ubuntu distros I tried! neither the ram usage or the constant cpu usage!
I also checked the versions which are very very similar for both systems: plasma 5.8, frameworks 5.26, qt 5.6.1 and amarok 2.8: how is that possible?

You should take it to that distribution, then, if there is a difference I very much doubt this has anything to do with Amarok itself.