If you are looking at things from only a dollar and cents point of view, then you will need to do a basic formula. If the cost of the restoration plus the amount you originally paid for the comic is going to be more than what the comic book will be worth, then restoration is not a good idea. However, if the cost of the restoration plus the original amount paid is less than the final value of the comic, then it could be a good candidate for restoration. Typically the older and more sought after the comic it is, as well as the worse the condition, the better candidate it is for restoration.
You might want to have a minimum amount in mind to make on such a comic. The minimum amount could be as low and a couple hundred dollars profit, or it could be in the thousands. The choice is up to you and only you can determine if the risk is worth it. Keep in mind that the market may not be able to bear the expected value. If put up for auction, there is always a risk that someone will not pay what you want for it, so the larger that improved value is, the better chances you have of making money off of your investment.
Now if you are looking at restoration from purely a passion perspective, then you will truly have to weigh the cost of the restoration verses the enjoyment of having a newer looking comic book. For some there could be no amount that they wouldn't mind paying for getting their favorite comic book restored. It is truly up to the collector to determine this.
Another aspect to this equation is that there is something of a stigma attached to a restored comic book. Again, this is reflected in that the value of a restored comic book is not worth as much as the same original comic in the same condition. For me, I would rather not own a restored comic as I really enjoy the original versions and consider the wear and tear to give it character. There is one comic that I would consider and that is Avengers #4. I have a big soft spot for this comic with it's iconic cover and would love a nice version of it. This is a very subjective matter that is truly up to each and every individual.
The bottom line is going to be what do you intend to do with the restored comic? If money is the motivating factor, than it should be an easy formula to figure out. If it is a comic you care about or have a connection with, then the decision could be harder as you have to figure out if the cost is worth it. Unfortunately that decision is up to you to weigh in the end.