Need suggestions for dual thermometer for cold frame

I would like a thermometer with a wireless sensor and a memory for min/max that I can put in my cold frame and with a sensor outside so I know what the temp differential is between the two. Any suggestions? I don't want to spend a lot of money but I don't want to waste money buying a poorly functional product.

Thanks

I bought mine at a local discount house. I purchased the one that is commonly used around a home.
I don't recall what it cost but it allows me to monitor the temps from the house.

I did find a pair of them won't work as apparently they transmit on a similar frequency.
I also found that I had colder and warmer areas in my GH.

I just got this 2-piece set from Amazon, after a LOT of research. It also measures humidity, which I wanted. Over the last 5-6 years I've had several kinds and this one so far works best for the price. It doesn't include batteries, and I suggest lithium batteries for the temp range.
http://www.amazon.com/TM005X-M-Wireless-Indoor-Outdoor-Hygrometer/dp/B004I08CEO