I live in England and the SSD/HDD configuration from Apple is even more expensive than it is in the US (change the $ sign for a £ sign...) Even with the expense of importing this kit from the US, I still came out with a super-fast mini for a third of the price.
It's allowed me to have the best of both: an incredibly quick boot drive and a large HDD for media storage.
The modification was very easy using the guide and took about half an hour, and I'm in no way a computer expert.
The only issue was that I was unable to reattach the two screws for the AirPort grill as the HDD's threads below ended up being slightly off-center. However, it all holds together without them and hasn't affected performance.
Unfortunately we suffer from painfully slow broadband in the UK so downloading Lion's Internet Recovery for installation took an age. However, once all 4GB of it was ready to go it installed in about 3 minutes! Then it was just a case of reformatting the HDD that was supplied with the mini.
Use the pronged attachment included in the screwdriver kit to get the tiny airport wire reattached as it's in too tight a spot to use your fingers to manoeuvre it into the right position.