You really can't guess what each person's cost is. It is totally dependant on too many variables.
I can't guess an exact amount, agreed, but we can do reasonable ballpark minimum costs. Let's use my location of Ajax as an example - but keep in mind that many of the GO riders we are talking about would come from even further out.
Parking alone will cost that much or more, I suspect. I'm not up on current parking prices downtown but even where I work at North York Centre it goes for $130-$140 a month, and there is no reason why it would be substantially less downtown.
Then, assuming you are driving a car that gets 10 L/100 km (to assume a fairly ballpark figure - many will NOT do anywhere near this well) and driving 50 km each way (basically, Ajax to the foot of Yonge), you will be paying around $40 a week in gas at current "low" prices. That's another $160 a month, totalling over $300 a month so far.
We haven't even talked about insurance (being able to tell your agent you are NOT driving to work will result in a significant discount, which should give you a few hundred a year in savings if nothing else) and depreciation and maintenance costs (50 km each way is likely something like 25000 km saved each year, which will have a very real impact). This is harder to quantify because a lot will depend on how long you intend to keep the car, but it's probably reasonable to assume that at the minimum it's a few hundred a month - possibly a good deal more. Of course, some will be intending to keep the car until it's near dead, but that just means you aren't taking direct a loss on depreciation so much as needing to replace your car a hell of a lot sooner, and that's a different cost.
And then - there is the cost of the person's time and stress levels. You're not going to get from Ajax to downtown during rush hour in 30 minutes. Even allowing time to park at the GO station and wait for a train, it's faster and less stressful. That's why people use it. This means that even if the costs were comparable (and they are not), a great many people will take the train anyway.
And finally - any significant divergence of people off of GO would increase travel times to downtown and increase downtown parking costs. There is a feedback loop here - as some people try to drive downtown, other people are automatically discouraged from driving downtown.
Obviously, everyone's math will be different - but it's clear that driving a significant distance to downtown is probably running you $500 a month or more. Yes, GO needs to be reasonable - but a $50 monthly parking charge as postulated above is not going to convince people to drive downtown - but because it's *more* than GO cofare rides to the station for a month ($25 in Durham), it *will* encourage some to take transit instead of driving. Not everyone, true - but enough to help.