how many should i have to spend to get steam lvl 20 to 30, and 30 to 40?

try this : http://www.steamcardexchange.net/index.php?badgeprices
or : http://steam.tools/cards/
I believe you need more experience each 10 levels to level up...
I usually spend like 3-5$ to get 10 levels =\

Here you go, if it's how much XP for each level that is needed you are courius about. :)
http://steamcommunity.com/sharedfiles/filedetails/?l=swedish&id=457523080

http://steam.tools/cards/

Scroll to the i on the right side and click Level calculator and Load Data. You don't have to be logged in but you might get more accurate results if you are because then it will take the badges and XP you already have in consideration.

i spent 88 $ to be lvl 56, always depend of the card you buy

You sure ask alot of questions.