Print your own Monopoly money!
My sister and brother and I used to play Monopoly, and often ran short on money. We remedied the problem with replacement bills that looked okay–straight edges, nice handwriting, not the same as the pretty colored cash, but you could pay the rent. Then, we had some crappy ass bills, cut to the wrong size, on lined paper with scrawley handwriting. The money was good, but somehow you never felt good with this money. When the banker wanted to rile a sibling, he or she (usually she since my brother was four years younger) would pull some crappy money from under the pile to pay when you passed go. The go-passer would try to argue for some better bills, and dischord would ensue.
We called the crappiest bills “Rotties” (because they were rotten,get it?). We called the better home made bills “Semi-rotties.” The money that came with the set we called “good money.”