In the past couple of years many companies have started offering "smart home", "market-ready, connected devices" as an "Internet of Things".  These are relatively inexpensive, easy-to-install, smart phone oriented devices for use in the home to monitor temperature, humidity, energy and water use, doors, motion  and control things like lights, pumps, outlets, appliances, etc.  Few of them are farm oriented but for a basic monitoring system they may be adequate.  Below are some I have researched but not yet tested.

SmartThings, (www.smartthings.com), San Jose, CA

In Aug, 2014, this company was bought by Samsung to become part of its Samsung’s Open Innovation Center group but claims it main its philosophy of being a "totally open smart home platform that brings together third-party developers, device makers, and consumers."  Can handle a wide variety of sensors.  Reviews indicate user interface is rather clunky at the moment.

Smartenit (formerly SimpleHomeNet), (www.smartenit.com), San Juan Capistrano, CA

Provides two gateway options ($160 - $200), repeater ($60) and about 50 different sensors and control units. Temp/Humidity is $70; solar radiation sheield is $50.  ZigBee, Insteon and WiFi protocols used.  No information on radio ranges on website.  Fairly good website but some details missing.  Pros: Wide range of devices available to control appliances, lighting, pumps, etc; asy installation; good smartphone software with ability to set triggers to control devices.  Cons: no temperature probe option, no radio range info on website, no other ag oriented sensors, no weather stations.

CAO Gadgets Ltd (http://wirelesstag.net), Irvine, CA

This looks like one of the lowest cost options.  From their website: "Wireless Sensor Tags and Kumo Sensors monitor and record motion events, door/window open/close events, temperature/humidity/soil moisture and water leaks. They can send you emails, notify you on your phone when temperature or RH is too low or too high, or each tagged item has been moved even only slightly, or when each door has been opened or closed. View graphs/data-logs of temperature, humidity, motion events at each wireless sensors anywhere with Internet. Attach to easy to lose items in your house and find them by beeping each tag from any browser or your phone. Out-of-range and back-in-range notification features come standard on all types of sensor tags. KumoApp Engine allows automation algorithms to be defined in Javascript to trigger Internet connected devices such as WeMo swtiches, Nest Thermostat and Phillips Hue upon events from sensors, schedules and geofences."  I have ordered these for testing.