Highest and average price are useless concepts.
The prices you see are not TRADE prices, they are simply OFFERS. And these offers are for the most part UNSUCCESSFUL offers. The reason you see them is that no one has been willing to buy at that price. The actual price at which most of these notes would trade, if they did trade, is BELOW the present offered price.
When looking at the prices shown in the API for some similar notes, the only useful calculation among these prices would be the MINIMUM. In general, the actual price at which trades would occur is below this value.