I was also looking this.
The TomTomOne can be bought for about £250.
It doesnt look as if it offers anything less than the TomTom300 apart from a newer,better looking design.
EDIT: Just found this on the TomTom website. It seems as if the only difference is battery life (TTone has 2 hours and the TTG300 has 4 hours) and the TTG300 comes with a carry case whereas the TTone doesnt.
If those are the only differences then i may well buy a TTone soon.
Here's what the TomTom site said was the differences:
So, which is better, the Go 300 or the One? Making a choice between the TomTom One and one of the Go range can be a tough one... here are the key differences, to help you decide:
* TomTom One: Smaller and slimmer than the Go range, with improved GPS chipset. Functionality is comparible to the 300 (Bluetooth / SD / touchscreen). Unlike the Go, the One is not supplied with a carry case, and TomTom quote 2 hours of battery life vs 4 hours with a Go 300. £70 cheaper than the Go 300.
* TomTom Go: Popular in-car navigation solution. The Go 300 is the basic model, the Go 500 supports hands-free calling with Bluetooth mobile phones, and at the top of the range, the Go 700 has a built-in 2.5Gig hard disk and supports door-to-door navigation across Europe