As Poppo mentioned, this would be only good for money launderers/plant thieves. But as **** as they are, they will need to be eta 1 or 2 to be actually useful
Plant Thieves should be able to steal planted plants (much like Arsonists used to be able to destroy them). That way you'd be guaranteed something even if it wasn't much. Money Launderers are useful but as you don't gain anything from it, people just choose Seed Thieves instead.
That could work as well, but the numbers that I have say even then it's kinda iffy. Here they are. Beware text wall.
Your average player is about 35% tree, 40% bush, and 25% mix of grass/flower. Grass produces a LOT of seeds (which are grown into a lot of plants). Grass by itself will probably constitute about 50% of your grab on average.
Lets look at the numbers. Plant thieves will take 300 plants. Lets say the current player is 30% tree, 45% bush, 10% flower, 15% grass. Now lets say they are at 1000 acres.
300 tree acres
450 bush acres
100 flower acres
150 grass acres.
Assuming the values on harvesters needed to tend each type is correct in the manual, I did calculations. I used a chart that shows how much money each acre will make per season, and the average in the year. I based the numbers upon those averages. I did not make mistakes such as forgetting to divide by 20. These are the numbers I came up with of how many seeds (and since seeds = plants, plants) each acre will make on a "season-less" tick, in which the annual averages are used.
483.75 tree seeds/acre
1,725 bush seeds/acre
5,340 flower seeds/acre
11,310 grass seeds/acre
now I'll multiply the numbers
145,125 trees seeds
776,250 bush seeds
534,000 flower seeds
1,696,500 grass seeds
So, to recap:
Assuming your target has 1000 acres, split up as 300 tree, 450 bush, 100 flower, and 150 grass, they will make 145k tree seeds, 776k bush seeds, 534k flower seeds, 1.7m grass seeds. Lets assume you went plant thieves and sent the correct amount of them. Even after the planting process, your thieves will walk away with about 55% grass. Some more math shows the issue here...
3,151,875
Per plant thief, you get get approx 12 trees, 72 bush, 48 flower, 168 grass on the average player. That will add up to about £21,420 on a £30,000 unit. These are just my trials, of course, but the message that its not only possible, but also quite likely that you won't reimburse yourself on 1 attack. Seed thieves would only get 1/3 of that, which I find to be quite too low, even though they usually attack inactives, etc and will be able to live through multiple raids if you do things correctly. Of course these are simply my opinions, and the numbers are what I use to back it up.