Do not try to lower, try to raise prices. I sell footage 1080p at 59$-79$ and 4K 199$, 189$(mostly), and all 4K down converted to 1080p at 79$. have portfolio of ~400 clips, and get 1-3 sales a month on P5
It is not much of sales, but it is not that big portfolio either, most of it is aerial-nature. And for now i think im ok, get some more sales on other agencies as well...
I tried lower prices at the begging - 49$ per clip, and no sales, later i raised prices and built portfolio and sales started. Trust me your sales are not based on your prices, but on your portfolio!
+ Pond 5 is more for video than photo, so your 13 clips is nothing (no hard feelings

If you lower prices, you do it for your self and for all the industry ultimately.