Try holding the Shift key when releasing the mouse button, you don't execute the command, only get an estimate cost. If your concern is accidentally releasing the mouse before finishing a drag, you could hold Shift until you're sure you've reached the right spot, and only then release Shift, then the mouse button.
Have to try the shift-key. Control-key works if one is draging a road/railroad - it works as a toggle between build/remove track. The esc-key is not very good except in special occasions, because it removes also the working instrument (f.ex. using esc-key while building i road you first select the instrument (a road-icon) and using esc-key returns the cursoir to normal when I only wanted to change the wrong beginning square of the drag).
Btw, why are signals exception from other dragging. The alt-key could just as well be used for flipping thru the types and the control-key should be reserved for a toggle on/off, (add or remove the signals) as is done when building a track. Is there any toggle key for switching add/remove for signals? (the same need for toggles is valid also for windows - F8 f.ex. turns on the finance report-window but it does not turn it off! A pity).