I will probably end up feeding a level 5 crit 4* to a mana 4* to give me 3 crit and 2 mana instead of 4-1.
Well both yes and no but not for the reasons you might expect. A 1* troop confers almost no real benefit when levelled but is relatively cheap to level up as is a 2* (IMO no one should waste resources on a 2* either unless you keep it for 3* tournaments AND then only keep the ones you have wasted resources on, plus lvl 1 duplicates). A 3* is even more expensive but far more cost effective in what you get for consumables.
With both 3* and 4* troops, as you get around lvl 6, the cost starts to increase dramatically (e.g. it does not matter what XP the feeder troop has, IT cost the exact same in food for each troop that you are using on that level!).
At level 1 on a 3* or 4* the cost of levelling up once might start at 300 XP - 450 XP respectively but each feeder troop may cost 5000 food (USING 3 troops as the example*) (1.5 x 3* feeder (200XP) = 1 lvl, cost ~7500); OR you might use 2* feeders worth 90 XP (3.3 x 90 (XP) = 1 lvl, cost ~15167(?)); OR worse, 1* feeders worth 40 XP (7.75 x 40 (XP) = 1 lvl, cost ~35750.
It is horribly expensive as each level costs more and more XP to level up; on a 4*, by level 10, it is 1500 XP per level (and the cost has risen to ~60000 per troop**) and you really don’t want to be throwing one stars at it now. It will take over 30 x 1(star; because of the damned embedded code creates italics!!) troops and about 2000000 in food for one level!!!
Yes, you only ever get the tiniest fraction of what you put in back so it makes using levelled up troops as feeders absurdly expensive. The lesson with levelling is to use your lowest value troops first as feeders until the cost becomes prohibitive and you work up to using 2* and 3* feeders (Naturally, keep 5 duplicates of each 2* troop (2 types), 3* (3 types) and 4* (2 types).
I haven’t read this but just found it. It probably explains far better than me:
Mana Bonus, troop levelling. Links in main post!!