I bought an
m-clip on eBay for like $40 in 2012 and have carried it daily ever since. Talk about value. It's rated for thirty bills, but it fits my license, credit card, debit card, player's card and maybe forty bills without a struggle. The great thing about it that you'll hear from other m-clip owners is it doesn't lose its tension even if you push its limits.
Honestly, if I found myself mentally preparing to throw a decoy, I would just hire a driver or ask security to escort me to my car. The score you are carrying should inform your risk model, but so should it determine your ability and willingness to pay for mitigation. Your plan depends on the perfectly incompetent mugger in your mind.