I think about 12 gold is the current price for a character slot (either as expensive 600 gems or "cheap" 800 gems), take or lose a few silver.
So if you are buying now to make a killer sale, it's too late. But if you need a character slot either way, it's not the worst price you can pay. Just don't buy it because it's on sale. Buy it because you need the slot.
Thanks. I purchased 3 character slots just so I have a slot for each profession and for extra mules. I'm kind of regretting it but I've always wanted 8 slots.
According to the wiki 8 slots is not the max. The max is currently unknown and someone has like 37 slots.