If you look at this from the idiom angle, well perhaps A through D are instant outcasts, for it is never possible to estimate something at some precise thing There is always an element of prediction, speculation, guesstimation, or approximation involved in estimation.
Therefore, 'estimate to be' 'estimate that' are better than 'estimate at'. Estimated at is more in tandem with locations and not with numbers at least.
E then is the sole survivor. But one can rest assured such an open and shut question is not going to appear in the reality of GMAT.
The essence of the argument is that if one or two choices had the idiom 'estimated to be' rather than 'estimated at', what would be the correct answer? I give below an edited version in line with the above thought.(I hope
MGMAT will have no objection for this practice session. )
The average individual’s cost of going into space is estimated at a fraction of the
price per economy ticket of transatlantic airplane travel by the end of the 21st
century.(A) The average individual’s cost of going into space is estimated at a fraction
of the price per economy ticket of transatlantic airplane travel by the end of
the 21st century.
(B) The average individual’s cost by the end of the 21st century of going into
space is estimated at a fraction of the price per economy ticket of transatlantic
airplane travel.
(C) By the end of the 21st century, the average individual’s cost of going into
space is estimated to be a fraction of the price per economy ticket of transatlantic
airplane travel.
(D) To go into space, the cost to the average individual is estimated to be a fraction
of the price per economy ticket of transatlantic airplane travel by the end of
the 21st century.
(E) It is estimated that by the end of the 21st century the cost to the average
individual of going into space will be a fraction of the current price per economy
ticket of transatlantic airplane travel.
We now have a 2 estimated at +2 estimated to be +1 estimated that. The question is wide open now to munch.