What indicator (or combination of those) could be used to roughly estimate the cheapness of a convertible bonds ?
Like the price/earning ratio for equities.
The above gives you the value of the option in the convertible bond. Next step, look at the price of an exchange-traded option to see what the embedded convertible bond option is worth. That can tell you if the convertible bond is over or under priced - compare the market value of the option to the implied price of the option (scaling prices to the same number of shares).