The GPS that comes with Streets & trips is a USB unit. It will NOT work with the Thera.

However, you CAN use the Pharos Bluetooth adapter with the MS GPS receiver to convert it to use with Blutooth enabled PDAs. But the price is expensive as you also need to purchase the Toshiba Bluetooth SD interface.

Look at the prices. Right now S&T is selling with a $10 rebate ($15CDN). My local Staples was selling them for $50 with an instant $15 rebate and the $15CDN mail-in for and end-price of $20!! (and Canadian to boot, eh). You can't beat that.
Now, look at one of the other threads here and you will find that someone on Ebay is selling the Holux serial unit for around $60US. team that up with the Thera and you have a good combo.
The only gotchya is the serial cable for the Thera. I had to build my own for which I've created a thread in this forum.

Hope this helps.