I think I am perhaps a little late to the party in this instance (I think I need to clear my cookies because the site kept logging me out even though I stated not to, and I was too lazy to log back in).
I'm surprised that you would like to replace your HG612s. Perhaps they are a bit old, but if it ain't broke...
Anyway, it is possible to source a Billion BiPac 8800NL on the bay for under £30 including delivery these days (used, of course, and they only come around every now and then). Heck, you can even get the 8800NL R2 (an updated version) for usually a little over the £30 mark if you have insane patience.
Both modem/router combos have a Broadcom chipset (BCM63168 for the former and BCM63381 for the latter) and have Telnet and SSH access to retrieve stats (and no unlocking is necessary). I've used the 8800NL with both DSLStats and HG612 Modem Stats and I can confirm that they are both compatible (and there is no reason with the latter revision should differ).
The only (perhaps minor) downside is that the aforementioned combos are not BT SIN 498 approved unlike the Zyxel devices mentioned by other forum users. However, I cannot see it being much of an issue because:
- As far as I am aware, there is no enforcement to use approved devices
- There should be no issue as the 8800NL & 8800NL R2 are using the same DSL chipsets as the Zxyel devices mentioned by other users (of course the implementation matters, but Billion is reputable)
I have used the 8800NL as a modem-only device and as a router, so I can confirm that it does a pretty decent job (and it's even better if you can find it cheap).
I am not aware of any other devices that could be cheaper, but perhaps someone here could make a suggestion if they know.