How much (estimated) would it cost to get this fixed?

Depends on year/make/model. New lock cylinder and having it rekeyed for old key (if doors and ignition are the same) and installed, i'd say about $100. If you don't car that the keys match or they are different for door and ignition, you could probably replace lock cylinder yourself for about $30.

I would cut one of the ground wires on starter there's 2 by main post and run a switch with wires inside car by drivers door low down so you can hit on by pulling on set belt - but now I know where it is - or lock smith 200 - 350

It needs a new ignition lock cylinder. the cost is usually not much unless you are driving a Saab or some of the older hondas and toyotas

Not sure just how much it will cost, but I doubt if it is more than $50.00. Take it to a lock smith not a dealership, you will get a much cheaper price.

I wouldn't bother - just lock your doors when you're out of the car.