Wal-Mart has several different styles of indoor/outdoor thermometers at a reasonable price, somewhere around $20-$30 or less.   Marty

We use a small electronic one, the base unit sits on the counter and we put the outside remote on a shelf under the slide. Works quite well, but we of course have to change the batteries every few months. As Marty says, Wal Mart has many models, around $20 if I remember correctly. Good luck,

I bought mine from Hannarvandtrailers.com, under Trailer Accessories.  It's large enough to read at a glance and sticks to the window, peels off when you move, and sticks again when you want it to.  I usually put in a north facing window so I don't get much direct sunlight that can skew the readings.  I also have a smaller indoor thermometer which I set up on the table or countertop.  This past summer at Lassen I got consistent readings that were 10 degrees difference between inside and outside, and I could tell from my comfort factor that they were fairly accurate.  The price is $8.99 plus S/H, which is the most reasonable I've seen.

gypsy

I just scrolled through all the trailer accessories on the link that I just quoted and didn't see thermometers. Use the search box for Thermometer, and it pops right up. gypsy

We use one of these.... http://www.nextag.com/LA-Crosse-Technology-WS-60666421/specs-html It came with two remote units so we use one outside and one in the refrigerator to monitor temperature. The clock feature is nice because it automatically resets to the correct time using the atomic clock signal. The weather forecast feature has been semi-accurate, it has shown rain forecast but only had clouds no rain. However a couple of times it's been right on for rain. Always right for sunny days or partial clouds. We mounted the main unit on the wall close to the TV so it's easy to see, the big numbers are easy to read from far away. It automatically rotates between the two sensors. My wife probably bought from QVC or HSN. I would like to upgrade to this: http://www.weathershack.com/honeywell/tn924w.html It's a full fledged weather station.

We also have a LaCrosse set with a projection clock/outside temp reading and two wireless remote sensors. I keep one sensor in a secured plastic container by the propane tank to get a close approximation of the outside temperature and another next to the water tank in case we are in freezing weather to see if the water is getting too cold. We have seen similar units from many brands at Wal-Mart, Target, ABC Warehouse and Costco.

If you decide to go with a weather station instead of just temperature sensors I can help. Weather stations will give you more information, ie. humidity, wind, rainfall. Some weather stations allow you to add additional sensors so you can have multiple temp sensors as other have mentioned. You can also add solar and uv sensors to some stations so you can get the whole picture. And, you can get software to run on your computer so you can track conditions. Just some ideas...weather makes a fun hobby...