The only thing stopping you now from buying a spontaneous plane ticket is your lack of spontaneity. Because no matter what your funds look like at the moment, there's a way to pay for it.
Airfordable, a new travel start-up, is giving travelers a way to buy their plane tickets using a repayment plan. Basically, it works like this: You take a screen shot of your flight details, upload it to Airfordable, and the site will give you a variety of payment plan options. You then select the one that's best for you.
Now they're not going to pay for your whole ticket upfront. You're responsible for a third of the ticket price, and a 20% transaction fee. Airfordable will take care of the rest of the upfront costs. You'll make regular payments, and when it's all paid off you'll get a copy of your e-ticket.
Basically, Airfordable is kinda like a high-interest loan, in that a 20% fee is high, but no interest accrues and no credit check is involved. Additionally, flights financed via Airfordable can't be over $500, which means you'll never be paying over $100 in fees. What happens if you don't make the payments? Well, you don't get your ticket, and that down payment isn't refundable, like most down payments. You can read all the details here.