Why is that? Price is known and discussed, client is happy, i'm happy. What's the problem?

I suppose you don't do services to clients? The most important part is that you don't undervalue your own services. See here some opinions of patio11:

https://training.kalzumeus.com/newsletters/archive/saas_pric...

https://twitter.com/patio11/status/463934642500804608

Do not price software at COGS! :-) - Perhaps the most important lesson in any development / selling gig. (i had to learn it the hard way, initial discussed price was too low, had to put a lot of more hours in to get even below minimal wage - once but never again)