Hey there. c:
Per item and Per hour are both good ways to judge cost. However, per item is calculated with price per hour in mind. An artist will go by experience or do a test piece to see how long any given asset will take to do, and then charge accordingly. Sometimes hourly is cheaper because the artist will find a more streamlined process while they're working to pump out a specific kind of asset. If they need to revise a lot because their first drafts aren't satisfactory, then per item is usually cheaper.
For isometric, you'd definitely want someone who's done it before, because the perspective is something the artist needs to tackle first. For the attack animations and stuff you'd just need to look for someone with relatively strong animation skills, a good animator can adapt to many different angles.
You'll want to ask the artists if they work part time or full time, that will influence the time range heavily. From my own experience, working half the number of hours on something per day doesn't mean it will get done twice as slow, in fact it's maybe 3x slower, but everyone has their own working speeds and habits. Personally, longer days are more productive overall.
For a true ballpark estimate you'd need to be very specific about what it is you really need. Is it a template, are there different body types or races? How many characters do you need done? How many different weapon animations?
Industry standard isn't as standard as you'd think, because client budgets vary so much... Some freelancers will work for as little as $10/hour if they are desperate, $20/hour is more or less standard for in house jobs, and $30-$40/hour is for higher budgeted projects. Some companies pay per frame, for example Wayforward (and incidentally the team behind Skullgirls, but those are hi res drawings/paintings) will pay $8/frame or higher.
For a self funded project you'd want to think about offering $15-$20/hour for the right artist if you can afford it. Once the standing proportions are all worked out you're looking for someone who can do a 8 frame walk in around one hour. Otherwise they're likely charging too much for their abilities and you should take a per item go of it because they don't entirely know what they're doing and need to experiment.
Hope that helps!