I imagine you're thinking about RFS (Reward Flight Saver) redemptions, which can indeed be extremely good value in some circumstances.
For many short European flights, they're available for 4,500 Avios on peak dates (less on off-peak) and a fixed cash fee of £17.50 - that's for one-way, in economy. For last minute bookings, or for those on extremely popular days, they can be a lifesaver when revenue prices are high.
At other times, when prices are lower, you may well be better off just buying a normal fare.
Of course, the one thing you will need is an Avios based account - and as you're in the US, perhaps you don't and this is why the value doesn't seem as good?