I have no problems with the price - I do agree though that it should significantly boost effectiveness. At least 25% is a good mark, I would think. I'd also think that if a sett has one and the attacking army also has one, the advantages that would be gained are eliminated, even if though the unit survives.
And just as another idea, eliminates night bonus effect.
That's a great idea fox! that would really make things tough for people who like to sleep

Thanks for commenting everyone, and these Ideas are great. Hopefully we'll get a few more answers then I'll re draught it taking your comments into account.
I's suggest that the proposed unit would be more biased towards the defender, as it raises the effectiveness of the whole army it's attached to, and with support in a settlement you can get more troops overall than any single attack. If the forces were of equalish value, then it would cancel out any gains technically