Literally: two – bundle – three – mon (an obsolete, very small unit of currency)
Alternately: Dirt cheap. Something so cheap that even selling it in huge volumes doesn’t make it profitable. Selling two pairs of sturdy woven sandals for only three mon.
Notes: Some versions of this saying use 足 – a standard counter for footwear – rather than 束, although it’s not clear which version came first. In any case, the phrase seems to be based on a real-world example of rock-bottom-price footwear in the Edo era, although probably not really for exactly three mon, since that particular amount can be found in a number of other phrases and seems to be a stock amount representing “cheap.”