Question: I signed a contract with my employer stating that for any and all reasons I am not working at that company before 2 years, I have to pay back x amount of dollars for the cost of training. Now this wasn't a small sum of money, but I got fired for reasons unknown (lack of "performance"). Is this common practice (and/or legal) in Ohio or elsewhere?

Answer: 
This site cannot speak to whether or not this is a legal practice.

I do know of at least one time this has happened in Ohio -- a person was let go, changed jobs, and the person's original employer wanted the person (or new employer) to pay for the training.

Asking employees to pay for training when they voluntarily leave is one thing, but it makes no sense when the company releases an employee.

Agreed that a brief meeting with a lawyer would be valuable. The lawyer would be able to advise on the legality and enforceability, which are often two very different things.

Employers sometimes do put essentially illegal clauses in contracts, since they know the employee would have to go to court and pay legal fees to get a judgement in their favor. Often it's easier/cheaper for the employee to go along with a contract clause than try to fight it. And very few want to be the person who sued their last employer.
You'd need to ask a lawyer about the legality. I have seen a lot of strange things in contracts and normally it's best to abide by them.

However once I leave a job I ignore anything the employer threatens me with. The last couple had sour grapes since many of their clients followed me out. Legal action is a costly exercise and I've never actually been prosecuted but I have been threatened twice. I wasn't sacked either time though, but I'd still ignore it if I was. This is probably your best strategy, don't answer emails, throw letters away unread, don't admit culpability for anything.

It would cost your employer quite a chunk of money (for a risk) just to get the process started against you. The longer you delay the more it costs them up front. If you're going to get prosecuted, then you're going to get prosecuted, there is no need to make it any easier or cheaper for the ex-employer and every reason to delay as long as possible.