So long as the alternative to the use of robotics remains human labor, the value of the work a machine does can be measured in terms of what it would cost to pay people to do the same work, plus the greater cost of transportation if that work is to be performed in a location which is remote from the market where the produce will be sold.
But considering only the labor component for the moment, if a machine is capable of working at approximately the same speed as a human worker, but can do so continuously, day and night, assuming the human is working a forty hour week, the machine can perform the work of four human workers, with eight hours per week left over for maintenance. If one person, working forty hours per week, can intensively manage one hectare, then a machine working at the same speed can manage four hectares (nearly ten acres) with the same degree of intensity.
If and when cultibots begin to become common, the comparison with a human performing the same sort of work will become less relevant, and the capacity of such a machine will most likely be measured in terms of the amount of land it can effectively manage according to a particular pattern, which requires a large subset of the range of operations such a machine might be able to perform. (As with any benchmark, such measurements will favor machines that are optimized for them.) And the value of that work will be the value of the produce at market, minus the cost of transportation to get it there, which, as now, will be largely a matter of what the human owner chooses to grow, and whether the weather works for or against them.
Given that such a machine might also be programmed to make a little room for native plants and animals, and that doing so would constitute a social benefit, the value of the work done might include modest subsidies for the land set aside in this way, probably in return for verifiable data that could be spot-checked for accuracy. At some point, once agricultural practices based on traction (pulling implements across the surface of a field) had become uncommon, the subsidies might be replaced with a simple requirement that a percentage of productive land, distributed in a manner designed to favor threatened and endangered species, be made available for native habitat. At that point, those still using tractors would need to bring in surveyors to stack off the parts of their fields they couldn't till, whereas those using cultibots would simply need to include the proper programming.