raiding C3s IS profitable, by about 1T+ plus each, if you know how to do it. you can probably make close to 2T, if you want to be tedious... i know for a fact this works, but the amount of work to squeeze 1-2T from a C3, really isn't worth it... unless you have no other way to make worker exchanges.
basically, you approach taking over the C3 with loosing as little resources as possible, and doing as little damage as possible. the 2 methods i use...
1. dec, and use your drone/fighter units, or nfp to clear the air defence, *only* you will not use these weapons again... then use lrd to paint, while your painting, use a bombers/drones to clear the capital, and the cities, *only* do not attack any thing else with the bombers. as the targets are clear, use your lrd to destroy the targets. work mostly on the cap, and cities, but hitting the town and counties to refresh the map is good.
this is the best method, it will leave 20 fort garrisons, all the bases, and factories garrisons. if you set them all to undefended, it will compensate the cost of the war, by over 1T. the biggest factors, are the NDM, and the MI.
option 1, is the cheapest, over all the best, but not always practical, as you may not be able to attack a C3 on your border
2. Drop in SF units, 3-5 of them. While your waiting for them to land, clear the air, same as above. then use bombers *only* to clear garrisons. the bombers will kill the mib and dmb, at little or no losses. then use cruise missles to destroy the targets. do this on the capital, and the cities *only* it'll take about 500 missles per city, and 1000 missles for the capital. the amount of bombs needed, if the country is not on your border, will be almost nothing. once the sf land, start painting, and use your missles to destroy a town, and refresh the map.
this method is more expensive, but by not destroying the ground troops, (using bombers and not drone, to clear garrisons,) you'll get more military hardware out of it... this method, is also a little more straight forward, as you don't have to manage having a ground unit near the target you are working on...
i've used both these methods, extensively, over 6 months, unless they made a bran new change to the assests of a C3, they will make you about 1.5t per country... but ofcourse to make a significant impact on your overall empire, it would take weeks of work, doing nothing but raiding. so it is very impractical. but to some one with more time on their hands, than they know what to do with, it can make you some ok money.