Level 1 mines cost 14000, take 8 seasons and earn 600 a turn (usually, in some provinces it is 1200 or even 1800, iirc; which also affects the output on level 2). This means, you have your money back in 8 + 24 = 32 seasons, which makes 8 years.
Level 2 mines cost 32000, take 20 seasons and earn 1500 a turn, so it takes 20 + 36 = 56 seasons (or 14 years) to get your money back, which is not that much in comparison to the length of the time frame.
I think Pella has one of the very lucrative mining regions with thrice the output of a normal mine and should be upgraded as soon as you can afford to (and own it, of course).