Step 1: Deny the conversation with a sip of coffee, and a dismissive shrug.
Step 2: Pretend you didn't receive the invoice, or better yet, accuse them of being cheapskates.
Step 3: Suggest a road trip to the store to "pick up a few things"
Step 4: Claim you accidentally spent it all on "research" (read: video games)
Step 5: Offer to pay it off in games of your favorite sport (you're 0-3 in chess)
Step 6: Say you need it for work, even if your "work" involves playing video games.
Step 7: Claim you just need a break from reality (and math)
Step 8: Suggest a group purchase of something expensive and shift the blame to your friends
Step 9: Offer to sell your kittens to make up for the difference
Step 10: Declare you are bankrupt, and start a support group for fellow bankrupt individuals