Or simply go to About this Mac in the Apple Menu> More Info > Storage and you will see a bar graph of your usage.
Despite what some may say 250 Gb is small. Certainly not enough for a primary computer if thats what it is.
In my experience media files are usually to blame and the metadata attached to them takes up a lot of the sector called Other.
Try moving movies, photos and music to the 1 Tb HD, BUT you must do each in the correct manner or you risk losing the lot.
You may then select the media you want to keep on the MBP leaving the full backups on your external. This of course diminishes the portability of your device.
The only other option is to upgrade your Mac HD to 500 Gb or more.
As an example I have my iTunes library which includes all my mp4 movies on an external HD. (all movies I import I convert to mp4 and delete the original formate version, eg, avi's)
That library comes in at 220 Gb. but I do have a lot of music. I am still using 340 Gb of my 500 Gb Mac HD on my MBP.
Your arithmetic is spot on, a 1 Tb external drive is a much cheaper option than a SSD upgrade. I still think that after 3 years as a primary device 128 Gb will be a bit small. For example I have 90 Gb of backups. These are such things as mobile device backups, icon backups, CS backup sync etc, etc, a total of 124 items. These mostly invisible files build up over time and the only real way to dispose of most (not all) of them is a matter of "Nuke and Pave" that is, a complete erase, clean OS install and restore from say a Time Machine Backup. Then I have 1302 cache documents ranging in size from 335 Mb to 12 Kb. So you can see that in three years a lot of memory is devoted to invisible process type files which are hard to find and dangerous to remove unless you know what you are doing.
Again, it depends how you use your computer. We are not getting into an argument because we are both right, for how we use our computers. I don't use the Mac for video editing, and use Google Drive for my image backup (along with DB). I use a Nifty MiniDrive with a128GB card for TM backups.