@eieio_31310
Mastery is damned expensive. You use coins to purchase each one percent increase in the Mastery level for each maxed out plant.
Once a plant reaches the max, you can open the Mastery option. On doing so, that plant will go from "Max Level" to "Mastery 1". The start position gives you plus 1% on "Damage Pierce" and a 1% chance of any plant placed on the arena receiving a Plant Food boost.
This does NOT apply to those plants while placing them in your initial Battlez setting up period. (I would imagine it would be the same on any 'Plan your Defense' levels in Adventure). Any plant placed on the arena AFTER you've initiated the match will then have that percentage boost chance.
Each 1% increase on the "Damage Pierce" scale costs "X" amount. "X" starts at 250 coins, but then escalates the further you go - 250, 500, 1,000, 2,000, 3,000, 4,000, 5,000, 6,000, 7,000, 8,000, 9,000, 10,000 every fourth increment - finally ending with a one off 20,000. At this point you will reach +50% "Damage Pierce" level. The coin charge then drops back to 250 before going through the escalation process again.
Each time you hit a +10% boundary on "Damage Pierce", the percentage chance of a plant being boosted when placed on the arena increases by one. Once you reach the +50 "Damage Pierce" level, the percentage chance of a boost has risen to 6%.
Now for the kicker - It will cost 243,000 coins to reach the +50% "Damage Pierce" boundary, then a further 243,000 to get to +100%. At that point the boost chance percentage will have risen to 11%.
I have no idea of how far the boost percentage will be permitted to go.
[EDIT] - It seems the Mastery level peaks at 200, so the maximum "Chance to boost" should be 21%. However each 5% increase will cost players 243,000 coins. To reach the 200 ceiling will cost 972,000 coins PER PLANT. (Excluding any coins players may win in Battlez, this equates to playing 1,944 adverts).
How far you progress is also limited by the number of seed packets over and above those needed to reach max level for any particular plant. (The ones that we kept getting regardless of the particular plant being at its max level). As you receive more seeds you will reach the next upgrade boundary. At which point you will then be able to purchase a further percentage hike for that plant.
Hope this helps, and hasn't put too much of a damper on any enthusiasm levels you may have been feeling.