Since you'll get your first blue you can ignore the mana increase for ranks 1 and 2. Rank 3 onward, the increase in mana cost means that for every three Crystal Slashes you did before this patch you now can only do two, or 1/3 less Qs, which translates to 1/3 less camps if you only take into account your Q damage.
But you don't just have Q damage, you have your autos. A Skarner ability/autoattack rotation, to be outrageously charitable to Crystal Slash cooldowns, was 2 autos and a q. Crystal Slash was only ever ~3/5 of Skarner's damage to clear camps, at most, averaged across camps.
So you're losing 1/3 of 3/5, or about 1/5 of your damage. So basically what the Crystal Slash nerf comes down to is that, when you have to base due to mana, you got one less camp for every 5 camps you did during that time.
Is this a big deal? Skarner already has god-tier speed coupled with S-rank ganking at 6. Hitting his sustained speed clearing by adding a harder cap to how long he can spam Qs seems perfectly reasonable. At worst his build options are decreased since you're incentivized to grab a sapphire crystal and go the triforce route rather than aegis or something.