Does price have a huge impact on the quality of road bikes? I want to buy a road bike but I have never had one. I have a limited budget and want to try triathlons. The good brands old bikes are really expensive. Even for a 25 years old bike I have to spend at least 450 Euros.

Surfing in the net, I found this bike on Amazon, which is in my price range. 

The problem is that when I have asked "experts" they told me that you cannot buy a decent bike for that price, and this isnt a good bike. Can anyone give an advice? Can this bike be good or is the price determinant for a good quality bike?
Search some more for secondhand bikes. EUR 300,- should get you a nice enough used bike from a couple of years back.

A tip I was given when searching for my first bike was to search the bike or racing bike category on ebay and similar sites for "ultegra" or the name of some other sufficiently high-end groupset rather than the brand. This filters out a lot of bad bikes, and a high quality groupset is very desirable in a secondhand bike because it is expensive to replace yet wears out very quickly (unlike (most) frames that are also expensive to replace but don't wear out so much). This search on the dutch e-bay equivalent gets you a few good options in your price range.

As to your question, I agree with Daniel R Hicks' comment that beyond about 1000-1200 euro's you are mostly paying for weight reduction (which might even impact durability) rather than 'just' for quality. I also agree with your "experts", you cannot buy even a descent new bike for that price, secondhand is a whole different story however.