You have to figure where the money would be coming from to figure out the appropriate bench mark, or in other words your own personal cost of funds.

If I was going to buy a $5000 bike it would come out of current income, or a money market account. I'm not going to take money out of longer term investments to buy a bike. So my opportunity cost paying cash is less than 1%.

Given that you shouldn't finance a bike, unless you could pay it off if you needed to, you're essentially looking at a short term investment horizon. The after tax rate of return for an investment portfolio managed for a 2 year horizon is deminimis today.

You could use the return on a longer term investment portfolio, but then you're effectively using margin financing to bet on stock market outcomes. Personally, the amount I'm going to invest in a long term portfolio is not going to be driven by how much Lynskey will loan me.