I think their costs only seem expensive when compared to their other shrekked counterparts: volksgrenadiers.
For 235 manpower and albeit only one shrek for 90 munitions, those volks retain a great deal of their AI. Then they start acquiring vet (up to two extra levels!!11) and scale wonderfully and durably.
Panzergrenadiers are not the same kinds of unit, and I can see how people who are used to volksgrenadiers' AT package might get frustrated/confused. They are best used defensively, as they are flimsy, don't get sniper level accuracy with extra levels of vet, and drop their shreks on retreat like crazy. Even though it is only 120 munitions for two shreks, a four man squad having two shreks is a recipe for allied shrek usage.
They are best kept in cover to protect your flank and/or your paks from armored rushes.
Are there more expansive weapon upgrades than PzGrens shrek? I never realized they cost 1-2-0 munitions