Phoenix Pick occasionally has discount coupon sales when you buy directly from them (usually announced in the newsletter) and they already give 15% off suggested retail on direct sales.
And if you're okay with getting a bunch of other sf "classics" at the same time, you can get
Lest Darkness Fall bundled in with a couple of other award nominee/winners for an amortized price of just over $5 per book (before applicable discount coupon). Still a little pricey, but DRM-free.
Unfortunately, there are no such semi-deals for the upcoming Gollancz releases.