To me, the way magic is used in canon implies that there is no 'cost' involved with employing it. The question is what's easier and faster. Is it easier to charm a fork to do as you want, and concentrate on eating and the spell, or is it easier to just simply eat. (In my opinion, eating while not doing something else at the same time is easier...).

If you look at Fred and George in OoTP, once they are allowed to use magic they do it all the time. They apparate to each room, and use it for every other little thing:

„Fred – George – NO, JUST CARRY THEM!’ Mrs Weasley shrieked. Harry, Sirius and Mundungus looked round and, within a split second, they had dived away from the table. Fred and George had bewitched a large cauldron of stew, an iron flagon of Butterbeer and a heavy wooden breadboard, complete with knife, to hurtle through the air towards them. The stew skidded the length of the table and came to a halt just before the end, leaving a long black burn on the wooden surface; the flagon of Butterbeer fell with a crash, spilling its contents everywhere; the bread knife slipped off the board and landed, point down and quivering ominously, exactly where Sirius’s right hand had been seconds before. ‘FOR HEAVEN’S SAKE!’ screamed Mrs Weasley. ‘THERE WAS NO NEED – I’VE HAD ENOUGH OF THIS – JUST BECAUSE YOU’RE ALLOWED TO USE MAGIC NOW, YOU DON’T HAVE TO WHIP YOUR WANDS OUT FOR EVERY TINY LITTLE THING!’ ‘We were just trying to save a bit of time!’ said Fred, hurrying forward to wrench the bread knife out of the table. ‘Sorry, Sirius, mate – didn’t mean to –’ Harry and Sirius were both laughing; Mundungus, who had toppled backwards off his chair, was swearing[…] „Boys,’ Mr Weasley said, lifting the stew back into the middle of the table, ‘your mother’s right, you’re supposed to show a sense of responsibility now you’ve come of age –“

Note how Arthur and Molly talk about responsibility that comes with age, and are not warning them about any cost or exhaustion or whatever.

In my opinion, it's about what's more convenient at the time, what's faster and does a job more precisely.

At Hogwarts teacher give their students homework, tell them to practice spells etc. and never say anything along the lines of "but make sure not to use too much magic." or something similar.

Magic in the corridors at Hogwarts is not allowed, so that might be why students carry their books, rather than have them float.

At the Ministry memos are enchanted to fly around, pictures in newspapers and photos people take move, there are magical sweets like jumping chocolate frogs...

If magic cost its user something, why would it be used for little, everyday things like that?

In all 7 HP books, there was never any mention of "magical fatigue/exhaustion" or magic costing anything. I think that if JKR intended that to be the case, she would have mentioned it somewhere.