CYDSTEPP is by far the most powerful glyph, there's no need to argue about that. It is one of the few ways to fight bosses with extreme damage (Avatar, Namtar, Bleaty) and it usually gives more bang per mana potion or explored tile than any other spell including HALPMEH, and it is the only glyph that gets you two extra attacks against any enemy at base mana due to precasting.
One of the things that make CYDSTEPP so good is that you can precast it. It doesn't just allow one extra attack with the normally available amount of mana, it allows two, or one in addition to whatever else you do with your full mana pool. Dispelling it on non-lethal attacks would make this less possible (it would still work on enemies that kill you from full HP). This would be a huge nerf of course, and precasting creates some interesting decisions. But eliminating precasting of CYDSTEPP is a much better nerf than increasing mana cost IMO.
Lets compare CYDSTEPP with other skills that get you an extra attack: GETINDARE and WEYTWUT. GETINDARE costs 3 mana and gets you one extra hit against enemies without first strike. Clean, simple and cheap. WEYTWUT works against enemies with first strike, but costs 7 mana and is blocked by magic resistance. Both of these extra attacks only work as killing blows. CYDSTEPP costs 10 mana, works against all enemies, and the extra attack does not need to be the killing blow. But you actually get attacked, which may cause weaken or other strategic effects. Also it is precastable, which means unlike the other two glyphs it gives you two extra attacks.
The comparison with HALPMEH is useful, but does not mention the advantages HALPMEH has over CYDSTEPP. First off, CYDSTEPP being the best glyph for getting many extra attacks is fine. One has to be the best, whether it is HALPMEH or CYDSTEPP does not matter. And HALPMEH has many advantages over CYDSTEPP. HALPMEH can get you an extra attack for just 3 or 6 mana if that is enough to let you survive a second attack. Also it quickly surpasses CYDSTEPP for resistant characters (with level 10 and 15% resistance it is often better than CYDSTEPP) and works great for wizards. It is also more suited to whittling down enemies while exploring, as it does not waste HP regen like CYDSTEPP would (unless you use BLUDTUPOWA).
Mana cost of CYDSTEPP affects at least four things: scalability (extra attacks per mana potion or explored tile), concurrent use with other glyphs (mostly GETINDARE), castability at base mana (which is 10), and use while worshiping Taurog. The only of these that might need a nerf is scalability (a bit stronger than HALPMEH in almost all cases, which as I argued above is fine IMO). However increasing mana cost would eliminate the possibility of using CYDSTEPP with GETINDARE for most chars, and would block Taurog worshipers from using it even earlier. Also most importantly it would block warlords from using it from the start, which is not good for a class feature. Same problem if the glyph is found early.
A possible nerf that only affects scalability is reducing the damage of your next attack after being saved from death (-50% for example). This penalizes repeated use but dodges the mana issues. It would also reduce the benefit from precasting, because the second extra attack would be weakened.
I think adding a strategic penalty to using CYDSTEPP or increasing the mana cost would be bad. If anything the mana cost could use a slight reduction, to encourage combining it with other glyphs. If mana cost is reduced scalability should be nerfed somehow of course.
I think dispelling CYDSTEPP on non-lethal hits, lowering mana cost to 8 and reducing damage of the next attack by 50% after being saved from death could work well. Reducing base damage of the next attack by 30% would be even better, as that would dodge all the issues with regular damage reduction (as seen with the alpha monk).