The first thing I ask myself before buying anything outside of my core business is "Can I afford to pay for this item if it doesn't produce a dime of revenue?" If I am comfortable with the payment amount, and confident that it WILL produce revenue, I'll take the plunge. If not, the idea goes on the back burner for future consideration. However, I never expect to re-coupe my total investment in 3 months in any business. Sometimes, it takes time to build a brand, and there are always additional costs of operating that you didn't account for.
As to paying for the purchase, the other option to keep the payments low is a Line of Credit. I purchased our VP-540 on a LOC at 2.75% interest, putting my monthly interest payment at around $35. If it's a slow month, the payment is minimal, and if it is a good month, I can pay down the principle without penalty.