EX should be much, much cheaper. But we're talking about pretty old technology. Yes, they have been raced for a long time, and there should still be a lot of old parts available. But you're talking about old brakes, old forks, old shock, old tires... All the technology is old. I bought an EX awhile back, thinking I'd track it. I took one ride on it on the street and said, NO WAY.
I think tops with a lot of engine mods you might be able to get 70HP out of it. But what tires are you going to run? Are you going to mod it, late model front end? Widen the rear wheel? Swingarm? Will the year you get even have disc brakes???
IMHO, I wouldn't do it. It's too much work to get the bike even remotely competitive, will cost too much in mods and have little resale value.
The SV right now is where it's at in this class. Tons of mods available, reasonable horsepower and options. Later and better technology at least availble for the suspension, wheels, etc.
If you're not racing, get a 600 and be done with it.