Frankly, it depends how much you use your bike. I commute these days with a used 150€ bike, for which I invested 100€ in upgrades. But that's because my commute is short (5km) and because it sleeps every day in the street next to a big train station. In the past I was commuting 2*10km daily and I had a 1000€ gravel bike that never stayed overnight in the street. It was definitely much more comfortable.

I would say a 300€ is fine for daily commute under 5km, but closer to 800-1000€ is probably a good idea for 10km and above.