In February, the company issued five-year bonds with a coupon payment of just 5.25%. The higher cost of borrowing has coincided with a 50% decline in SolarCity's share price this year. Higher interest costs should alarm Tesla's investors, who will inherit SolarCity's obligations if a proposed merger with Tesla wins shareholder approval. Just as ominous is that it looks like outside investors aren't willing to buy up SolarCity's debt, despite the higher yields. Given the bullish global backdrop for bonds, a company offering an interest rate as high as 6.5% on such a short maturity should be beating investors back, if all were well.Tesla shareholders ought to keep that in mind as they weigh whether to approve bringing SolarCity in house. Photo - SolarCity said Tuesday that Chairman Elon Musk and other top executives would together purchase more than 80% of the solar energy company's latest $124 million bond issue.