Definitely do-able. Cost prohibitive? That is not up to the peanut gallery to decide. Trees are not inexpensive. Planting, and replacing them, is not inexpensive. Just for fun, with a bunch of assumptions:
A single 1000' roll of 1/2 main line tubing for drip systems runs approximately $130 (dripworks.com). 40 acres, assumed to be square in shape, would be 1320x1320. A single run would then be $172, just in tubing if you go wall-to-wall, which you probably wouldn't. Assuming the trees to be in placed in a grid fashion, multiply $172 by the number of rows plus another $172 to connect all the rows to the main line. At this point, the acreage is the cost driver, not the # of trees. You will need
an emitter for each tree.
A bunch of 4-way and a couple of 3-way splitters.
Couplers to splice together the 1000' rolls where appropriate.
Zip-ties to terminate the main lines.
A 10-lb pressure regulator.
Optional:
timer(s).
Maybe Optional, Maybe mandatory, depending on the number of emitters:
A manifold (splitter) at the hydrant to maintain enough pressure throughout the entire system. The site I get my stuff from (dripworks.com) has some guidance as to how many emitters is too many.
Installation is super-easy. A kid can do it. Goofs, including design goofs, are easy to remedy.