I can get 10 Wemos D1 minis delivered for $20 and 30 cheap soil sensors for around another $20. (Chosen for wifi connectivity which bare Arduino lacks. One micro can monitor many sensors, but most don’t want a bunch of wires everywhere.)
It’s often the case that it costs more for the power source than the micro and sensor.
These aren’t Arduino brand arduinos just like your office xerox isn’t necessarily made by Xerox.
the sensing side is reasonably easy, but the automatic watering gets hairy. OP was proposing one pump+reservoir for each plant, which adds up the cost and makes powering the system more demanding. And you probably want some sensing in the reservoirs too so that they don't run dry. Alternative would be to do plumbing to create a water network, but that brings its own challenges especially in residential setting where plants are scattered all over the place.
AliExpress or anywhere else cheap electronics components are sold. (Haven’t looked, but I’ll bet dx, taobao, banggood, and the others also all have them. Amazon and EBay are often faster and cheap-enough now that ePacket has been nerfed. Search something like “soil sensor 10pcs”; looks like they’re $12-13 for 10 there.)
They’re cheap to make (basically just a small PCB with a few passives and a connector) and while they don’t last a long time in some soils, they’ll last long enough for you to learn what you want and then decide if you want to change sensors every year or three, go higher quality, or find another way to estimate evapotranspiration and water based on open loop control and monitoring (which is what most people likely end up doing, either technically or by human approximation).
If you (or anyone else) is in, or willing to go to, the edge of Cambridge, MA, I’ll set a couple aside for you for free.
