Sruk,
I don't think you should be too quick to label this guy a "bastard" and be so angry about this.
It certainly sounds like he was trying to take advantage of you, but maybe he has no idea how long this work takes. And, on top of that - there is the fact that *you agreed to the deal*. I think you should be careful how you approach this because it's bad form to agree to do something, then become angry at the other party, then refuse to do what you agreed to do. That doesn't mean I think you should do it for $125 - but I think you should consider the reason you accepted in the first place. Was it because you really had no idea how long it would take? Were you too shy to ask for more money? Or were you willing to do it for $125 knowing how much work was involved?
I think your best bet is to either tell the guy that you took a look at the project and realized the scope is much larger than you expected and it will take a lot more time than $125 will cover. Or, you could tell a small lie and say you didn't understand the PCB needed redone, and you thought it was just some minor adjustments, or something.
There are a lot of people who have no problem using others - this guy sounds like he may be one of them. Personally, I would not offer someone $100 for a 10 carat diamond ring, but some others would do that and think "hey, if they accept then it's their choice and their problem if they are unhappy". You should take away some life experience from this event at a minimum.
If this guy has disputes with other suppliers then I would suggest coming up with a price you would be "very" happy with, tell him you got some other work that came along and this is more work than you realized, and tell him you will be unable to do the work unless he can commit to a much higher price.
He'll probably be quite upset - especially because his personality is probably the "love to get something for nothing" type, and thinking he just got an amazing deal only to have it disappear is likely to lead to anger.
Just figure out your price up front and stick firm to it. In the future, if anyone ever offers you something but you feel pressured to say "yes" to a bad deal, always just tell them you will think about it and get back to them the next day. Then you have time to think it over without the pressure.