Cost is going to have to go way up more than that...that's practically nothing for all you want.
75/1000....extremely unfair, bring it up a lot. It shouldn't be a high cost relative to what you have now but high relative to what the total is.
After all you have...high damage if hit directly while drastically decreasing their speed, then if you slightly hit them, they get slightly slower, yet still receive moderate damage from the spell itself...so, take that into account and raise the MP cost.