How much could I expect to pay for a transponder designed for this purpose?
There is no radio frequency transponder with a built-in sensor designed specifically for determining the moisture content of soil, though there are generic moisture sensors that could be used for this task. RF Code offers a tag designed for leak detection, for example, which might work for your needs. I don't know the cost, but active tags with sensors are generally priced at around $50 to $75 apiece.
In 2012, we published an article about passive moisture and corrosion sensors being deployed within a parking garage built in the German city of Boppard (see German Parking Garage Installs Moisture- and Corrosion-Sensing RFID Tags). The passive tags with sensors were placed below the cement's surface and above reinforcement steel at select spots throughout the garage. The tags can be read using a handheld device to detect the onset of corrosion or excessive moisture, thereby enabling the city to repair the problem before it can harm the reinforcement steel (see Has an RFID Solution Been Developed for Detecting Moisture?). The entire system cost just $10,000. This might work for your purposes, as long as you do not need to take readings over a large area (passive tags have a short read range).
There are also wireless mesh networks that could be fitted with moisture sensors. Mesh networks forward data from one node to another until that information reaches a node connected to a computer that hosts an application for interpreting the collected data. The advantage, with such technology, is that you could install these over a large area and have readings performed at regular intervals without a need for any human involvement. A company called Libelium makes such sensors for agriculture (see ZigBee System Makes Parking Smarter in Spain).
—Mark Roberti, Founder and Editor, RFID Journal