I think cost calculator doesn't work well for lower stacks i also lost my sixth ship in encounter, it was about 36M encounter and my ship had about 8M power, which means i lost 8M ship to 6M fleet, this never happened to me before and i do missions that have power equal or a little greater than my first ship, it happens that i encounter single fleet but i never lost my main ship.
This couls also explain what happened to lamma - wrong calculated tiny man fleet at fourth stack.

lol
i tried several times boarding in mission but the result was always self destruct  so i though that you cannot aquire ships this way...it seems i had simple bad luck

PS.
it's working