This is variable, there is no normal fee. It depends on timeframes, how far, how much etc,. Some consultants charge the hours travelling (myself included) and take their travel expense out of that. Others charge the travel expenses separately.
You seem too emotionally involved.
Frankly, I think that continuing to lingering in this hate and vendetta fantasies for the company is not healthy for you; I perfectly get that unfair treatment and being fired is terrible, but you have to move on, emotionally and professionally.
Yes, you could play some tricks on them, but you have a commitment as a ...
Companies often have you pay and then reimburse you to minimize this behavior.
Interview practice is always good. Just go, use the interview as free practice, and then enjoy your time in the city complication free. If they offer you the job, just decline. This strategy would almost assuredly avoid legal problems.
There might be nuances using which you can figure out something to avoid the legalities for avoiding the interview and use that company-provided fund to have a personal tour, but being very straightforward, I'd suggest : Don't do this.
Since you already mentioned:
Since they won't stop harassing me no matter how many times I tell them to screw off