My prior data said most designs were being done well above 28nm (even 90nm) with mask costs increasing as the feature size got smaller. eASIC even still offers 45nm S-ASIC's (Nextreme-2's) for those who can't afford ASIC's at that node. So, I'd be surprised if 28nm cheaper. If 28nm has gotten that cheap, then I'd love to have references on that to give to CompSci folks I've seen doing semi-open or FOSS designs from 90nm-32nm.
Note: I'm also talking for complex designs like a CPU or SoC instead of Bitcoin-mining primitives or something.
Edit: see my response to wmf as I partly answer thoughts on your other question about volume or eating up costs.