@Sergiusz **********
All extensions are 1900 baht.
You can apply for an extension when you have 30 days left at most offices, and when you have 45 days left at Bangkok and Chiang Mai.
Jomtien you can only apply the week before your stay is up.
Tourist and visa exempt extensions are mostly a formality and you will not be rejected if you have all the paperwork.
Immigration won't ask to see your plane ticket but your airline might not let you board if you don't have proof of travel out of Thailand within 45 days. You will need to ask them their policy.