An understandable dilemma. Disc brakes, no rack mounting holes on fork, $100 budget.

First, consider ways other than a front rack to carry your stuff.

Second, get the Jandd disc adapter. It definitely allows the rack stay to be bent out beyond the caliper. You might be able to cobble something together from the hardware store, but this adapter does it without compromising load bearing capability.

Third, rack fitting is always a crap shoot so one often has to try different brands until you can settle on the best one for your purposes. So buying from a source where returns are easy is important. Mounting at the caliper hole is not so common, so there might not be someone here who can make a confident recommendation.

I'm afraid that your $100 budget will be consumed quite rapidly. If you can relax this constraint a little, call Wayne at the Touring Store. He may have some suggestions.