I was given an Amazon voucher so didn't really shop around -- it cost me £32 with postage. I've seen them for slightly less on eBay. I found a 'pre-order' price listing for the 12GB of £75, but I'm sure that'll drop quickly, especially after higher capacities are announced. Really it all depends on how long you want to wait.
The only slow-down I've noticed is in initialising the database, which unsurprisingly takes somewhat longer when it has more tracks to scan. The post-initialise restart takes about 20 seconds with approx 12GB of tracks (normal restart is probably slightly slower too, but not enough for me to notice). Apart from that, it's business as usual.