I'm really scratching my head trying to work out what the rationale for the sunflower' field's balancing is.
You can buy a maximum of three. One with 8k beanstalks, two for 2.5k gems each.
Let's start by talking about the cost. 2.5k gems is more expensive than any of the Improved buildings with the sole exception of the Improved Deerstalker. It's slightly more than a Friary, far more expensive than an Exotic Wood Tree Nursery, and almost 3 times the cost of a Recycling Manufactory. All of those other buildings produce an output that can be used in multiple chains. The Sunflower Field produces oilseeds which are used only in the Oil Mill.
It cycles once every 90mins, turning 10 water into 1 Oilseed and can't be upgraded. I figured it might be possible to get one to basically feed a Titanium Hen, but that's not possible even with the 10x buff that you can apply for the bargain price of 200 gems a day. The Hen will consume 240 seeds in the 6 hours that the field takes to make 44 seeds when buffed. Even all three runnning buffed can't keep up with a single Hen.
Ignoring the Hen, the field just ... isn't that valuable. If you had all three and you ran them unbuffed, you're getting 48 seeds a day for the low, low price of around 30 Euro in gems and whatever the rate for 8K Beanstalks is on your server (On Newfoundland, 8k Beanstalks is around 60k gold coins). 48 seeds, a full day of production for all three fields, will feed a single oil mill for 4 hours. I figured that maybe the plan was to reduce the seeds from adventures to balance the new inputs, but even with all three fields running, you can't even come close to keeping an oil mill running even with rainbow snow buffs on all of them. If you have multiple oil mills, then it gets even worse.
If you do adventures, even casually, you get more oilseeds than you can use. Why would we spend any number of gems or beanstalks to get this tiny extra trickle of a not very valuable resource?