I'm surprised more folk haven't suggested this, but another possibility for those seeking a (relatively) cheap, and HD-capable, HDMI display, is a small flat-screen TV.
I've looked around plenty of places at cheap monitors, and I don't think I've seen any with HDMI ports until the price gets into the £100-150 bracket. Beneath that level, you'll be lucky if you find a DVI port on them, and the cheapest only sport a VGA socket. Even if you find a lower-cost DVI monitor, they don't always handle sound.
On the other hand, visit almost any big supermarket (Asda, Tesco, etc.) or electronics store, and you can usually find a 16" or 18" flat-screen TV for as little as £65-70. The quality may not be top-notch (though most that I've seen working don't look too bad), and "label-snobs" might bristle at the typical "A.N.Othercom" badge on the front, but these TVs usually give you at least one HDMI port, and often an extra £20 or so will get you a DVD player built-in.
I don't have one of these TVs myself, but only as my disposable income isn't even that disposable most of the time
All the same, it seems to me like another option to consider for Pi-folk seeking a cheap HD display.