What's the average cost of getting level 100 on Steam?

google fu 175$

200€

Using those "level up services"?

Thats not what you think. Its the same as just buying the cheapest sets.

Do you really get daily trading cards at what level?

Source? You will not get any daily cards -.- But you get booster packs randomly. On level 120 and nearly 7k games I get 3-5 booster packs each week.

The cheapest option: Buy individual cards for small sets (5-7 cards) for cheap games (< €0.08 per card) and you can usually get one set per €0.3, €0.4 at most.
To reach 100, you need roughly 10 x 1+10 x 2+...+10 x 10 = 550 sets (from level one), so roughly <€160-200 should do the trick.
Assuming you already have existing cards to reduce the amount you need to buy, you can cut down a considerable amount of costs.
The expensive option: Buy complete sets from level up services. These usually cost more per set since they're precollected, but can be profitable if you value your time, since hunting individual cards can quickly consume a lot of time. Compare prices to the cheap option when making your decision.

100 is easy to get for free if you spend bit of time instead.

Uhhh I'm over 100 and I'm lucky to get a pack a week.

I received about 30-40 booster packs in the past two years and I have been over level 100 the entire time.
As for the cost, if you use some of the cheaper level-up bots, then about 50-55 CS:GO keys at best. If you use the market well and have some luck with booster openings, then somewhat cheaper.

Depends, do you have multiple bot accounts to farm free giveaway games and event cards? If yes, you can get there without wasting too much money, I remember only wasting 3 to 5 csgo keys buying sets.