This is a serious issue for you bud.
Basically, the advantage you've gained by going down the affordable housing route has been eroded by the lower valuation in reality.
From your post, it sounds as if you're paying around the 270k figure, which in monetary terms is a 50k saving, when you're being told it's a 100k saving.
If indeed you are 100% sure you'll not be selling in 20 years (and living there the whole time) the valuation issues wont effect you, but I urge you to do as much as you can to get the house valued closer to the real market value, because it'll bite you in the bundy in the long term if you decide to sell...
Good luck with it...