Anyone knows how can I calculate how much power did PSU lost with some time like 1 year?
good PSUs generally don't degrade all that much. As long as you are within your warranty period and are within your wattage limit, you should be fine.
There is some debate on how much a PSU may lose over time. Most of the PSU's will suffer a failure in the 5-7yr range. Electrical components have come a long ways, and there are many examples of PSU's in the commercial world lasting 10's of years without failure.
Personally, I would look for a PSU that is in the Tier 1 range (http://i.imgur.com/tgrbCnr.jpg), and with a good warranty. Why a good warranty you ask? Because the longer the warranty is in place for, the more the company has faith that their components will last. They either have to fix it or replace it. Either option is an expensive solution for rolling out a budget PSU with poor quality parts. that being said, I would highly recommend the EVGA G2 750W or 850W as you can see EVGA's warranty below from their page.
This product comes with a 10 year limited warranty with registration within 30 days of purchase. (http://www.evga.com/Products/Product.aspx?pn=220-G2-0750-XR).
It's a bit problem here, cuz I live in Croatia and parts are really expensive so I have to buy used power supply about 1 year, that's why I asked is there any lost power here or not.
I was looking for Corsair CX750, it's not the best, but I think it's good for r9 280x and FX 6300
If you check out that link in my above post, it'll list the various Tiers and the PSU's contained in them. Then you can cross-check what's available in Croatia that will fit in your budget.
As for loss of efficiency? Somewhere in the 1% to 7% per year on average.
The Corsair CX series is in the Tier 3 (Acceptable) range. Shouldn't be an issue, however around year 4 of its usage, I would think about replacing it. It'll get the job done though.
Here you need