I really like this idea actually. I personally haven't spent money on keys, but I do have one in my possession (through trading) and I would definitely do this, even just for fun.
However one addendum if I may. Let's say only 10 people entered the raffle, then one guy could spend $25 dollars to buy 10 entries and have a fifty- fifty shot at doubling his money. seems like it could be taken advantage of.
Your idea might work better if you did something smaller let's say a simple craft hat as an entry and the pay out was 20 craft hats. However I have no experience in this field.