I've stumbled upon this situation: the Emissary Bor'kan (the middle one) comes with Missiles, Gravitic Hooks and Railguns. Those Gravitic Hooks bring forth 2 Wardens, which is somewhat implied on its cost (193 - (37x2) = 119). However, if we field a Emissary Sa'cea (the bottom one) and 2 normal Wardens... the total cost is 186! It's a 7p discount that makes the Emissary Bor'kan completely useless. You could say 'the escort limit', but it's so large it hardly matters.
Here's a pic for better understanding:
Emissary Bor'kan should cost LESS than a Emissary Sa'cea and 2 Wardens, because then there would be a point on fielding one...