Depends on what use you're putting it to. it may be that 8gig is more than enough - it certainly is for me.
fwiw i bought an 8gig kit from Amazon UK.
16gb wil certainly future proof it to an extent. Amazon has 16gb kits for the Mini
http://tinyurl.com/8w8trv6
£75
As I say I have the 8Gb kit - which was £32
http://tinyurl.com/8t5er8z