Cost = (M*M)/20 + 2*M
(where M is the number of mins you need to complete the build - doubled if you don't already have the first 10 mins). The game rounds this down to the nearest integer if you buy the full amount; if you buy a partial amount you have to round up the fractional ec's.

If you are looking for things you can conceiveably remember or maybe do in your head, there is a simpler approach which waste a little, but not too much and is easy to remember:

If you need 10 mins (1 standard row), it costs 25 or 2.5 each;
if you need 20 mins (2 standard rows), it costs 60 or 3.0 each;
if you need 30 mins (3 standard rows), it costs 105 or 3.5 each;
if you need 40 mins (4 standard rows), it costs 160 or 4.0 each;
in other words, for each extra row, the unit cost goes up by 0.5 ecs.
If you just pay that amount, you will always have enough (unless you don't have the 1st ten yet) and will only waste a few ecs.