I agree it intuits as being slot cost heavy and I am wanting it to feel fun instead of poof, technically if you are fairly certain the counter spell will work with my suggestions it is slightly more powerful doing low but potentially flavorful whiplash damage on the controller of the countered spell.
Even changes some are suggesting like making it opposed checks I think is flavorful mechanics not really attempts to weaken it.
My concern with many of the suggestions offered in this thread is complexity. The more rolls you add to a rule, the longer that rule takes to resolve. Since CS is a reaction, it really needs to be quick. Even the roll for for higher level spells is stretching the time taken to resolve this, so making it an opposed check is making things worse, not better IMO.
A whiplash/feedback mechanic seems like it could be a good, flavourful and fun idea but I am not sure it will be worth it. Especially if passing the CS check hits the original caster and failing hits the counterspeller. It's a net 0 effect (not mathematically I know).
As it stands casting CS runs the risk of using a 3rd level slot for no effect, which strikes me as a good risk/reward mechanism. When it comes to outright countering spells 3rd level and below I think that skipping the roll for a faster resolution is a simple way to keep things going when the effect of the countered spell will likely be minimal.
To me things look like this:
PC level 5/6: You're using your top level slots to lock down an enemy is a HUGE cost so should have big rewards - guaranteed success:
PC level 6 to 10: Using 3rd level spells is still quite a big cost so countering low levels spells seems fair while countering higher level spells comes at the risk of failure (a spell slot for absolutely nothing). Using higher level spells to guarantee success is trading power (high level spell slot cost) for certainty.
As you increase in level lower level spells become more trivial so I like seeing a quick resolution , while keeping some risk or cost when throwing out big guns.
The only change I would really think to be good would be to require the CS to be of a higher level that the countered spell. ie Casting CS at 3rd level would auto counter 1st and 2nd level spells. That way casting a 9th level spell always has a chance for success.