This is average for a DIY system like that. I have 5.4 kWp solar with 3 Victron MPPTs, a 15 kWh battery, and a MPII-3000GX, and I spent only 3.200 €.
I bought 540 Wp solar modules for 49 € each. For a Yixiang DIY battery case I had to pay about 420 €, and for 16 cells EVE 280K I paid 1.100 €. The 3 MPPTs did cost about 350 €, and the MPII-3000GX was 470 €. The rest was for installation material. Meanwhile the cells are available for 900 € only.
Here in Germany this is the normal price. For over 700 you can get the MPII-5000GX.
This offer is not even the cheapest: Victron Multiplus II GX 48/3000/35-32
I did not fancy going for a DYI battery …. that is indeed where you can save a little more but as mine is mounted outside (shed) I needed wall mounting, IP rating and heating build in …. no way I would manage that building it myslelf.
Where is the problem? The new Yixiang DIY box even comes with a fire extinguisher, and heating the battery really is no problem. If I would need to place my battery outside I would build a case with insulation around it, and place a simple terrarium heater and a 10-€-thermal-switch inside. All together less than 50 €, and really a no-brainer.
Can you share where you ordered the case and cells?
I ordered the case over Alibaba, and it came from a store in Poland. The cells I ordered over Ebay from this dealer. Cells with the plain stud are availabe at only 850 € here. These prices include transport, isolation plates, busbars and screws. EVE MB31 cells with 314 Ah have almost the equal price, and they too fit into the Yixiang box.
For more info about the Yixiang DIY box see Youtube, especially the videos from Andy (Offgridgarage).
BTW: My cells are almost perfect: After I built my battery I wanted to do the first time top level balancing, and used a laboratory power supply. But when my battery reached 55.2V for the first time the cell difference was lower than the 10 mV which I did set in my BMS to start balancing. And now, after almost one year of use, the cells are still very, very close, this is a video I did made just now:
True. I bought a MPII-5000 for less than €700 including VAT (for our cabin in Norway) so it could have been even cheaper otherwise when installed in Germany. Also got some MB31 cells directly from EVE for €1360 including shipping (from a larger batch). Turned off balancing in the BMS to see how well they work. And after five months. All is good.
In Germany for solar systems smaller than 30 kWp there is no VAT. So the prices that I mentioned here are all without VAT.
Please, where did you find this?
Because Victron pricelist states 711€ (without VAT !!) and most dealears stick to that or even put on top…
I could use idealo.de and then find these:
Unfortunately they don’t allow links to offers, my and other’s answers have been deleted without any notice. Simply use Google to find offers starting at 377 € for a MP-II-3000, or at 456,50 € for the MP-II-3000GX
€470 is in germany and taxfree (mp2 3k)
In NL they are availble at €505 incl tax (mp2 3k)
Or €625 incl vat for a mp2 3k gx
i bought the mp2 5k in NL recently for €700 incl tax
Indeed…
In RO, for a mp2-3k-gx, 700 euros, all taxes included.
An mp2-5k-gx is about 1000 euros, all taxes included.
And a Multi RS, 2000 euros, all taxes included.
And these are among the smallest prices.
Rule no.10: Greed is eternal… 
How much tax? In Germany you can get the MP2-5K tax-free for as little as 549,07 €
Tax is 21% here, so without tax its €578