<1 euro game to get to 30euro order

https://isthereanydeal.com/game/superflight/info/
https://isthereanydeal.com/game/islanders/info/

Plants vs. Zombies GOTY Edition is a great option if you're looking for something quite cheap and also pretty good :)
There's also other options available, but I'm not sure how much you're willing to spend (within the cheap range ofc).

EDIT: Nevermind I just noticed you're looking for a <1 euro game. My bad :)
Also, Sonic Generations Collection is really good as well.

I'm in the same position right now, considering maybe adding this one: https://store.steampowered.com/app/388620/DungeonUp/
Seems pretty neat if you like turn-based roguelikes

https://store.steampowered.com/app/268770/Treasure_Adventure_World/

Unbundled and positive reviews!

Civ 3 is a game that I adore and have sunk over 2K hours into, and is only 1.24 Euro
Another amazing game is Sid Meier's Pirates, but it is 2.49 Euro atm.

I got "Sonic Generations Collection" myself.

Alternatively, you could also buy one of the cheap DLCs, for games that you already own.
For example:
One of the 0,99 DLCs from Borderlands Bundle
or
XCOM the Bureau DLC for 0,49