When I have spare P. primuliflora plants, I sell mature, flowering size plants for £3 at CPS meetings, which is only $5 US. It isn't a rare species in the UK, I often see plants produced by the large Dutch CP nurseries on sale in garden centres for about £5 ($8 US). Compared to the other five SE USA Pings, it's the commonest by far. If your being charged £10 (this is $18 US folks!!) for this species, then in my opinion you are being ripped off! I doubt that the wholesale cost of these plants is more than £2.
My most expensive Ping purchase to date was P. conzattii which I paid 15 Euro (+5 Euro postage) for a single plant, which is a little under $25 US in total.
I recently bought the Cuban species, P. filifolia, for 20 Euros ($25 US), but did receive quite a few plants and shared the cost with another UK grower.
I've been given or traded (swapped) the majority of my Pings, so the cheapest are free!
Vic