It was three or four years ago last I was given a quote, at that time it was $90 per pound plus shipping.

Plant potassium in sap is always low until the very end of the season, as it always gets outcompeted with sodium. There’s no boron in the water but the soil has strong native boron and it is always high in the sap. Boron is not my limiting factor in K uptake, it’s sodium.