The most frequent discount rate I have observed in Silicon Valley has been 20%. Now I compare that number against the loan interest rates for risky SMBs at about 36% APR. Or even the 18%-24% APR factoring rate against inventory for SMBs with little or no credit. And, in stark contrast to startups, these SMBs should have some cash flow and probably has been operating for years. So the note discounted rate of 20% is a relative bargain for startups with little or no revenue history.
Then you have a valuation cap which projected value is anticipated by the maturity date of the notes, somewhere between one and two years. Basically the company is providing a ceiling valuation for the note holders. Now I have calculated valuations. But this magic number is purposely chosen from low figures that should satisfy the risky nature of the notes. Of course, if the number is too high, then the note holders just get a fair exchange. And if the post-money valuation skyrockets, then the note holders have the benefit of a real bargain.
But as one can see, there are yardsticks to establish a price. It boils down to the cost of capital and the financial projections. There is a saying on Wall Street when establishing sales projections: under-promise, over-deliver. So in the case of the valuation cap, you are always better off to price the discount at the most pessimistic value. And once one sees what community banks or lending clubs charge for interest rate, the discounted rates seem to be a bargain.