I've not bothered to install a thermometer in my current boil keggle. I also removed it from my mash tun keg. I'm using thermal probes/sensors connected to my Fluke 52ii thermometer to get temperature readings. For me, the thermometer in the mash tun was getting in the way of mixing the grain in. In the boil keggle, it wasn't all that useful. You can tell, visually, when it's boiling. I also use the Fluke setup as I'm chilling, since I do a recirculation through my plate chiller before sending it to the fermenter. The sensors on the Fluke give me a much better reading, so I can better decide when to switch over. I also have a sensor/thermometer at the plate chiller, so I know what the chilled wort temperature is.
I would advise getting a waterproof, long stem, digital thermometer for use. That way you can use it in more than just the boil keggle. One like
this would do the job.
While I do like the Blichmann products, thermometers included (they look sweet) I [also] cannot recommend them in good faith. Especially since it seems like you have a tight budget.
BTW, you can use the thermometer I linked to above with the
fitting sold by Brew Hardware to use it in the keggle. I have one of those (from a different vendor) in the assembly that comes off the wort out side of my plate chiller. It allows me to send the long stem thermometer all the way to where the wort is coming out of the chiller. I can also use different o-rings and use one of the sensors connected to the Fluke thermometer.