Amadren: Here (France), Internal hard drives are 40-50€ while external ones are 80-90€
ET3D: I took a look at Amazon.fr and it seems like you're wrong.
That really depends on what you search for, I just searched and:
WD Green 1tb internal = £38.99
WD Elements 1tb portable = £44.99
OP:
Anyways, from what I have heard around the drives in external are not as good as internal ones, and via USB your not going to get SATA speeds.
But again that is also subjective, if you need storage you carry around a lot, then portable one may be best. If you have hot swap drives like me, then I would say internal is best. Also, different types of drive are better at certain things, i.e. ones that last a long time are good for backups, really fast ones are better for daily use - you wouldn't use SSD for backup would you?