I can't speak for the others but he.net doesn't seem to interested in customers making use of their "dual stack" network. We looked into getting IPv6 space from them to go with our IPv4 assignments for a couple of racks of servers in one of their datacenters. They wanted to double the monthly fee for data and drop a second v6 only port to our racks, not my idea of a "dual stack network". Needless to say, we do not have native IPv6, a few of our customers that desired it are using HE's free tunnel broker service though.
It is possible that they deployed a second group of routers to do ipv6 rather than bet it all on a dual stack. Heck I've seen engineers at various places digging 7507's out of the dumpster in hopes of getting their company going on v6. HE.NET has been doing ipv6 for a long time now so it is (also) possible that their deployment pre-dates a 75% bug free IOS version for dual stack. I can't comment on any sort of pricing going on. Rumor has it that some transit and hosting companies are indeed selling IPv6 today (and surprisingly enough actually have customers buying it). -Scott