The answer really depends on how you want to use it. Do you want to view the data remotely, or are you OK with driving to the site and looking at statistics, such as yesterday's numbers?
Various sensors might mean different things to you. Do you need to know rainfall, as in "how much" and "how long ago"? Do you need to know wind speed and/or direction? Do you need to know how much sunlight? Are you ultimately looking to determine soil moisture? Or do you just need to know if it's above freezing?
If cost is the overriding factor, I'd suggest starting simple by using http://www.weather.gov
. It's the National Weather Service's official site, they have the most accurate forecast information available, and it's free. They have radar loops available as well as rainfall maps so you can see exactly what's happened at each location, and you can view either by zip code or latitude / longitude coordinates.
If you're OK with driving to the site (or having an on-site employee read you the numbers) and don't need more than the standard temperature/humidity/wind information, you can probably pick up an inexpensive La Crosse station at a local garden store for about a hundred dollars or so.
If you need the more sophisticated measurements, you may want a more flexible / expandable system. A lot of us have Davis Instruments Vantage Pro (or Vantage Pro 2) systems, which are pretty good "amateur" stations. They're sold for use in agricultural and industrial applications, too.
You can get Vantage Pro packages that range from the standard temperature / humidity / rain / wind gauges, you can add solar radiation, UV, soil moisture, leaf wetness and soil temperature sensors. You can add a solar power kit if you don't have electricity on site. You can also add a telephone adapter; there are farmers who have cell phones hooked up to their remote sensors and dial them with a modem to retrieve the data.
If you want the data from your station posted on the internet, you'll have to have a remote way of accessing the weather station plus a computer. With the Davis systems, you can use the cellphone/modem combination I mentioned above and post the data from your office computer, or you can have an on-site computer connected to the station console that has its own internet connection. That also means an extra cost for the software and network service, but is certainly the most flexible.
And I don't mean to imply that Davis is the only game in town. There are other home systems that could work well for you; and at the other end of the spectrum there are commercial / professional units used by meteorologists, researchers, airports, etc.
As with anything interesting, you can spend any amount of money you want. Check back here if you'd like recommendations of specific weather station vendors (we all have our favorites), and ask around on other boards.