A challenge? I accept!
Let's be time efficient! You get 10.000 food in one minute for 500.000 coins. This means you need to grow sarjin peppers 131 times for a total of 65.500.000 coins. You need to do this 4 times to level 20. So that's 262.000.000 coins. Yikes!
Let's say you breed the most expensive dragon for the fastest time. Air dragons. You can breed in 2 pens for 1.000.000 every 2 hours. Assuming you play all day and take a minimum of 8 hours rest, that's 8.000.000 a day. This way it'll take you just under 33 days to max it out.
Let's say that you can earn about 1/3th of that by visiting people and cashing in your habitats. It still takes you 22 days. Even if you could conjure up ridiculous amounts of coins, I'd still be very surprised to see you do it in 1-2 days!
Now just growing treats then. You need to grow them 524 times. You have a maximum of 11 treat farms.
It'll take you a minimum of 48 minutes to grow them all. More realistically going to 60-90 minutes. Not too bad considering you have 22 days to grow them
Of course, this is without considering gems as a resource...
Still ridiculous considering this is to level up 1 dragon from lv 19 to lv 20

EDIT: Just for giggles, leveling up a dragon from level 19 to 20 using treats you buy from the treasure shop, it would cost you 5700$

Also, considering it doubles every level, you could calculate how much leveling a dragon from lv1 to lv20 using bought treats costs: 11.400$
Happy shopping all!