Are imperial levels worth building?

This response no longer applies after the 1.3.0 update

The rewards for imperial floors are calculated as follows:

((M1 * Q1) + (M2 * Q2) + (M3 * Q3) * 51)

Where M1 is the number of minutes it takes to stock an imperial floor and Q1 is the number of items stocked from that floor.

M2/3 and Q2/3 are for subsequent floors required to redeem the reward.

Basically, for every minute it takes to stock an Imperial floor item, you get 51 credits.

E.g., one of the missions is:
15 Rebel Secrets
1 Salvage
2 Battle Plans

The total reward is 13,515 and is calculated as follows:
Rebel Secrets = 10 minutes to stock
Salvage = 25 minutes to stock
Battle Plans = 45 minutes to stock

((10 * 15) + (1 * 25) + (2 * 45)) * 51 = 13,515

To answer your question about if the rewards increase, yes. I have 5 Imperial floors, and the current mission I'm on awards 18,040 credits. This does require 12 different items, so 12 supply officers are needed. I could buy them all for 12,000 to speed it along, as waiting it out has taken some time.

You definitely get more credits than the rewards the emperor gives you, but whether that's worth the payoff to build the levels I guess is up to you.

I currently have 73 levels out of 80, my imperial levels are 16/16.

Before they added an extra 6 levels there were only 10 which lead all the way up from interrogation to communications, I remember that I received 250k credits every time I got the required items.

I do recommend that you build them. They will help you start out with a lot of money at low death star levels (Please note that when I received the 250k credits, I was 80% done with the quests)